Posilac

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Medically reviewed

Laura Arias

Last updated on 22/12/2025

This page provides general, reference-level information compiled from official medical sources. It is not a substitute for professional medical advice, diagnosis, or treatment. For decisions about your health, please consult a qualified healthcare professional.

Overview of Posilac

Quick Facts

Property Description
Active ingredient Somatropin beta (Sometribove)
Form Sustained-release injectable solution
Pharmacological class Somatotropin/Growth Hormone Class
Common use Increasing milk production in dairy cattle
Origin Synthetic, Recombinant DNA Technology

Posilac: Defining the Synthetic Protein Analogue

Posilac is the trade name for a veterinary drug whose active component is Somatropin beta (Sometribove), classified within the Somatotropin/Growth Hormone Class of pharmaceuticals. The active substance is formally known as recombinant bovine somatotropin (rBST), a manufactured protein analogue structurally identical to the natural bovine somatotropin hormone found in cattle. This compound is produced using recombinant DNA technology. The specific Posilac product is primarily targeted toward professional dairy operations in regions where it is approved for use. Milk and meat from cattle treated with a bovine somatotropin product are recognized as safe for human consumption.

Composition and Form: The Sustained-Release Injectable

The product is formulated as a single-ingredient preparation containing rBST suspended in a specialized aqueous or lipid-based vehicle. This results in a sustained-release injectable solution, designed for administration via subcutaneous injection. This dosage form is a distinguishing feature, engineered to facilitate the slow and steady release of the active hormone over an extended period. This mechanism allows the pharmacological agent to maintain a consistent presence in the animal’s system without requiring frequent dosing. The administration of rBST to lactating cows increases their efficiency in converting feed into milk.

General Purpose: Supporting Lactation in Dairy Cattle

The fundamental purpose of administering the active ingredient is to utilize its potent lactogenic effect to sustain and prolong the period of peak milk production in dairy cattle. By mimicking the natural growth hormone signal, the substance induces a controlled metabolic redirection, shifting the cow’s internal resources and energy primarily toward the mammary gland for milk synthesis. This process supports and assists in maintaining a higher volume of milk yield during the latter stages of the lactation cycle.

Regulatory References

  1. FOI Summary - Posilac (rBST)

What side effects are possible with Posilac?

Possible side effects and safety information

The safety profile of Sometribove (Posilac), based on official regulatory documentation for veterinary use, is characterized by an increased risk or increased incidence of various health disorders, rather than standard frequency classifications like common or rare.

Adverse effects are documented across multiple system-organ classes in the treated animal:

  • Reproductive System: May be associated with reduced pregnancy rates, increased days open, and retained placenta.
  • Musculoskeletal and Connective Tissue: The regulatory data indicates potential for an increase in foot region disorders, enlarged hocks, and lesions of the knee.
  • Immune/Infectious Disorders: Use is associated with an increased incidence of clinical mastitis and subclinical mastitis, which are significant safety findings.
  • General and Gastrointestinal: Other documented effects include digestive disorders, periods of reduced feed intake, and an increase in body temperature.

Localized and Population-Specific Safety Notes

Injection site reactions are a documented finding, frequently presenting as temporary swelling that may begin three days after injection and, according to labeling, may persist for up to six weeks. Safety considerations are also defined for specific groups: first calf heifers may experience a more pronounced increase in the risk of certain conditions, such as increased days open, while older cows may have a greater incidence of foot region disorders.

Safety Restrictions

The product label includes a specific Human Safety Warning noting that the product is not for human use and that repeated skin contact with the protein may produce an allergic reaction in some individuals. Use is strictly limited to lactating dairy cows.

Overdose and Emergency Response

The official regulatory guidance for Posilac focuses on the risks associated with accidental human exposure to this veterinary product. The product is strictly not for use in humans and must be kept out of reach of children as mandated by regulatory authorities. The overdose information is explicitly framed around potential contact rather than systemic overdose from ingestion.

The primary documented manifestation of accidental exposure risk is the potential for an allergic reaction in people. This risk is specifically linked to the fact that the compound is a protein analogue, and frequent skin contact with similar protein-based substances may sensitize the user and produce an immune response in some individuals. These warnings apply to any person handling the injectable solution.

Regulators mandate specific, immediate actions to mitigate exposure risk. These procedures require that hands and skin exposed to Posilac must be washed immediately with soap and water after any handling event. Furthermore, any clothing soiled with the product must be thoroughly laundered before it is worn again.

Immediate medical attention must be sought if any signs or symptoms of an allergic reaction occur or develop following accidental exposure to the product. The official prescribing information does not list specific requirements for hospital monitoring or observation following such an event, nor is a specific antidote detailed in the labeling for accidental human exposure.

Therapeutic Uses of Posilac

What Posilac Treats: Main Uses and Benefits

This supportive intervention is used in situations involving the symptomatic pattern of natural decline in milk output that is generally characteristic as the lactation cycle advances. The product supports the management of the expected reduction in yield, contributing to the benefit of sustaining a high volume of milk production for a longer duration than may naturally occur. This may assist with maintaining the overall output trajectory during the relevant mid-to-late lactation phase.

The product is generally applied in contexts marked by physiological limitation of production efficiency, a situation where the animal's internal metabolic energy may be sub-optimally directed toward milk synthesis. The relevant therapeutic areas of use generally involve: managing the decline in milk yield, supporting lactation maintenance, and enhancing milk yield efficiency. The use of this agent may assist with managing this challenge, supporting the practical benefit of enhanced milk yield efficiency.

“This intervention provides supportive relief for the challenges associated with sub-optimal production output and assists with the overall management of the physiological demands of lactation.”

This targeted support may assist the animal with maintaining efficient conversion of feed intake into milk, thereby providing supportive relief for the challenges associated with sub-optimal production output in high-performing cattle within professional management scenarios.


Quick Fact: Relief for Production Decline

Property Description
Therapeutic domain Supporting lactation and sustaining milk volume.
Target condition Natural decline in milk output during mid-to-late lactation.
Symptom management focus Improved milk yield efficiency and maintenance of output.
Clinical context Applied as part of professional production management strategy.

Regulatory References

  1. Animal Drugs @ FDA

Eligibility and Restrictions for Use

Posilac, a brand name for the drug sometribove zinc suspension, is an animal drug approved for use in healthy lactating dairy cows to increase milk production. The drug is administered by injection typically starting around the ninth week after calving and continuing until the end of the lactation period.


Approved Target

Category Notes
Healthy Dairy Cows Specifically for increasing marketable milk production in cows that are currently lactating.

Exclusions and Contraindications

Posilac is not for use in humans. Furthermore, its use in cattle is subject to specific health and management considerations. The manufacturer's label advises that cows using Posilac may have an increased risk of conditions such as:

  • Mastitis (udder infection)
  • Reduced pregnancy rates and increased days open
  • Lameness (foot and leg problems)
  • Injection site reactions

Its use is generally not recommended in cows that are already experiencing severe health problems, and it is contraindicated for purposes other than enhancing food production, as its use is limited to the labeled indication.

What should I know about interactions with other medicines?

Official Interaction Information for Posilac (Sometribove Zinc Suspension)

Regulatory documents concerning Posilac (sometribove zinc suspension) focus on the potential for increased needs for veterinary treatments and the procedural constraints associated with administering other medicines.

Unlike many pharmaceutical labels that detail direct drug-drug interactions, such as changes in drug clearance or enzyme effects, the official information for Posilac does not explicitly list pharmacokinetic or pharmacodynamic interactions with specific medicinal products or product classes. There are no stated restrictions on combining Posilac with specific veterinary drugs based on a mechanism of action conflict.

Contextual Constraints and Therapeutic Drug Use

Official labeling advises that cows administered Posilac may require more therapeutic drug treatment for mastitis and other general health problems compared to untreated animals. This increased need for therapeutic intervention creates an important procedural constraint related to food safety.

Constraint Area Regulatory Requirement/Note
Interacting Categories Therapeutic drugs for mastitis and general health problems.
Procedural Constraint Adherence to the withdrawal period of any therapeutic drug administered is mandatory.
Marketability Impact Milk produced during the required withdrawal period for any therapeutic drug must be discarded.

This constraint ensures that only marketable milk, free from drug residues above established FDA tolerances, enters the food supply. Therefore, while no direct drug-drug interactions are defined, the use of Posilac is officially tied to the required management of withdrawal periods for any subsequently necessary therapeutic medicines.

Mechanism of Action

Receptor Agonism and Endocrine Cascade

Posilac (recombinant bovine somatotropin or rBST) functions as an agonist for the Growth Hormone Receptor (GHR), activating the intracellular JAK/STAT signal pathway. This binding rapidly leads to the amplified production and release of Insulin-like Growth Factor 1 (IGF-1), contributing to a systemic endocrine response. The molecular interaction initiates a mechanistic cascade within the somatotropic axis.


Systemic Metabolic Repartitioning

The IGF-1–driven cascade triggers a homeorhetic effect, which is a coordinated, long-term adjustment in the body's metabolism. This mechanism preferentially shifts the flow of energy substrates, such as glucose and fatty acids (via lipolysis), away from tissues like fat storage and toward the milk-producing cells of the mammary gland.


Physiological Activity in Mammary Cells

The mechanism facilitates the mammary epithelial cells' priority access to circulating nutrients while simultaneously stimulating their synthetic activity. This action supports the elevation of the cell's metabolic rate, which is the underlying physiological change that contributes to the resulting systemic effect.

Dosage and Administration Information

Posilac (Sometribove) is administered as a sustained-release injectable suspension, exclusively via the subcutaneous injection route. The administration follows a fixed cyclical pattern consisting of standardized timing and dosing.

Standard Administration Protocol

The foundational principle of use is the intermittent, bi-weekly administration of a fixed dose throughout the lactation cycle of the dairy animal. The protocol involves a single, fixed dose of 500 mg of sometribove zinc per cow, which is repeated every 14 days (bi-weekly).

Feature Standard Instruction
Dosing Schedule A single, fixed dose of 500 mg per cow.
Frequency Injection is repeated every 14 days.
Route Subcutaneous injection only.

Contextual Usage Timing

The administration is tied to the animal's physiological state. Treatment begins during the mid-lactation phase, defined as 57 to 70 days after calving. Use continues on the bi-weekly schedule until the end of the current lactation period. The product is for use in healthy lactating dairy cows, with no population-specific dose adjustments specified for this use.

Procedural Requirements

To ensure correct delivery, the injectable suspension must be allowed to warm to room temperature (between 15 C and 30 C) prior to administration. Instructions state that the subcutaneous injection site must be rotated between the cow's left and right sides on consecutive injections. This detailed procedural structure defines the standardized approach to using the medicine.

Recent Clinical Evidence

Research evidence / Overview of studies for Posilac

Evidence for Maintaining and Enhancing Production Output

The primary research for Posilac has been applied in contexts examining production output and lactation persistence in dairy cattle. Studies have been conducted using established scientific methods, including numerous Randomized Controlled Trials (RCTs) and large-scale meta-analyses that combine data from multiple controlled settings. This research was evaluated in contexts involving the natural decline in milk output that typically occurs during the mid-to-late phases of the lactation cycle. Researchers measured key outcomes related to systemic or functional imbalance, focusing on the volume of milk produced daily, as well as the efficiency with which the animal converts feed intake into milk.

Across the evidence landscape, studies monitored patterns related to the volume of production throughout the duration of the administration period. The studies monitored changes in the output-to-input ratio, or feed efficiency, and data show patterns related to these efficiency metrics observed during the treatment interval. These findings describe group patterns observed in the studies and contribute to the broader evidence landscape that is considered when assessing the intended use. The research reflects the specific conditions under which these controlled studies were conducted.

Analysis of Milk Composition in Clinical Trials

Research was also applied in studies examining potential changes to the final milk product itself. Researchers examined specific outcomes such as the concentration of macronutrients (milk fat, protein, and lactose) and levels of certain hormones or growth factors, particularly Insulin-like Growth Factor-1 (IGF-1).

The findings related to milk composition describe patterns where the measured concentrations of macronutrients were generally within the usual range of biological variation found naturally in non-treated cattle. While some research highlights changes measured during the study period, studies monitored these reported shifts and noted them as transient or minimal in some reports.

Duration of Studies and Long-Term Follow-up

The typical follow-up durations for the pivotal controlled trials were intermediate to long-term, designed to cover the full expected treatment phase, which often spans approximately eight to ten months. This approach allowed researchers to observe responses over defined time intervals that covered the full administration period.

However, there are still limitations in the evidence base regarding very long-term outcomes. Data for certain groups remain insufficient when considering the impact across multiple, successive lactation and reproductive cycles. Limited information for long-term outcomes extending over several generations of animals is not fully established.

Key Studies & References

  1. Freedom of Information Summary: Original New Animal Drug Application for POSILAC (Sometribove)

Frequently Asked Questions (FAQ)

Common questions about Posilac (FAQ)


Q: Is Posilac considered safe by regulatory bodies?

Regulatory documents indicate that the U.S. FDA approved Posilac based on a standard of 'manageable risk' for the treated animal. Additionally, multiple authoritative bodies have concluded that the milk and meat from cattle treated with the product did not pose a risk to human health related to consumption.


Q: Does Posilac interact with common over-the-counter products?

The official labeling for Posilac does not describe any specific pharmacokinetic or pharmacodynamic interactions with common human over-the-counter products. The constraint noted in the official documentation relates to the mandatory withdrawal period for any therapeutic veterinary drug administered to the treated animal.


Q: Is Posilac different from other treatments that seem similar?

Official product information describes Posilac as a synthetic protein analog, specifically recombinant bovine somatotropin (rBST). This substance is classified in the Somatotropin/Growth Hormone Class, which defines its mechanism of action and distinguishes it from non-hormonal or chemically different treatments.


Q: What does the official documentation say about Posilac and breastfeeding?

Official documents include a strict human safety warning that states the product is not for human use. Its approved purpose is entirely focused on increasing milk production in healthy lactating dairy cows.


Q: If I am taking a supplement, could it interact with Posilac?

The official labeling does not specify any pharmacokinetic or pharmacodynamic interactions involving human supplements. The core interaction constraint described in regulatory documents relates to the necessary adherence to withdrawal periods for any therapeutic veterinary drugs administered to the cow.


Q: What is the risk classification of Posilac according to government agencies?

The U.S. FDA approved the product based on a standard of 'manageable risk' concerning the animal's health. Official documentation describes potential adverse effects not with a single risk code, but by summarizing the increased risk or incidence of specific conditions like mastitis and reproductive issues.


Q: Are there different forms or strengths of Posilac available?

According to official product specifications, Posilac is formulated as a single-dose, prolonged-release injectable solution. This product is available as a single, standard-strength formulation of sometribove zinc suspension.


Q: What should a treated animal generally experience when starting Posilac?

Expected outcomes described in the labeling include a cyclic response in milk production that is greatest during the middle of the 14-day administration interval. Treated animals may also exhibit an increase in their voluntary feed intake over a period of several weeks after administration begins.


Q: Can Posilac be used in conjunction with blood thinners?

The official regulatory information does not list pharmacokinetic interactions with specific drugs like blood thinners. The use of Posilac alongside any therapeutic veterinary drug is subject to the strict regulatory requirement that the drug's mandatory withdrawal period must be followed.


Q: Are there specific age restrictions for Posilac according to the official label?

The official label strictly limits use to healthy dairy cows that are currently lactating, which defines the appropriate life stage. Specific safety considerations and findings are documented for certain subgroups, such as 'first calf heifers' compared to 'second lactation or older cows.'


Q: Are there any rare but serious side effects associated with Posilac mentioned in the official data?

The regulatory documentation lists potential adverse effects by describing an increased risk or increased incidence of certain health disorders, rather than categorizing them based on frequency (such as 'rare' or 'common'). These documented effects include mastitis and reproductive issues.


Q: How quickly does the effect of Posilac typically begin?

The effect on milk production is described in official documentation as being cyclic within the 14-day administration interval. The response is generally observed to be greatest during the middle of each period, with the overall goal being a sustained long-term adjustment.


Q: Is it true that Posilac is used differently in other countries?

The U.S. FDA indication for Posilac is to increase marketable milk production in dairy cows. The product's regulatory status has been reviewed by numerous international bodies, leading to varied decisions regarding its approval and use across different countries.


Q: Can Posilac affect the results of general blood tests?

Official documentation for the treated animal notes that use has been associated with changes in certain blood parameters. Specifically, regulatory data includes reports of reductions in hemoglobin and hematocrit values during the treatment period.


Q: What if I forget when I last used Posilac?

The administration protocol establishes a 14-day interval between uses. It is important to note that the regulatory documents do not provide any specific instructions or procedures regarding how to handle a forgotten or missed administration.


Q: Are there specific food or drink types mentioned in relation to Posilac use?

Official labeling includes guidance on 'Nutritional Management' for the treated animal. Documentation advises that cows increase their voluntary feed intake and recommends that dairy operators avoid sudden dietary changes while the product is in use.


Q: Is Posilac a prescription-only product?

In the United States, regulatory labeling classifies Posilac as an Over-the-Counter (OTC) Animal Drug. This means that its distribution is not restricted to prescription by a veterinarian.


Q: Is Posilac available under different brand names globally?

Posilac is the registered U.S. brand name for the active ingredient sometribove zinc suspension (rBST). Since the product has been reviewed by international bodies, other brand names for the same active ingredient may exist in different approved regions globally.


Q: Does Posilac cause any noticeable changes immediately after use?

The expected effects on milk production are described as cyclic and long-term, not immediate, with changes in feed intake occurring over several weeks. One noticeable change that is documented in official warnings is a localized injection site swelling, which may appear temporarily after administration.


Q: What are the major themes of safety research on Posilac?

Research and regulatory reviews have primarily focused on several key themes regarding animal health outcomes. These themes include the product's association with mastitis and reproductive performance, as well as its impact on milk composition in areas like Insulin-like Growth Factor-1 (IGF-1) levels.


Q: How do regulatory bodies monitor the ongoing safety of Posilac?

In the United States, the FDA initiated a Post-Approval Monitoring Program (PAMP) specifically to track the product's safety profile. This ongoing program was established to track animal health and milk safety outcomes following the commercial introduction of the product.


Q: Are there any common reasons Posilac use is stopped?

Official regulatory documents define specific conditions under which the product's use should be discontinued, such as when average daily milk production drops below a specific regulatory threshold (not due to temporary illness), or if injection site swellings repeatedly open and drain.


Q: What are common concerns people have about the ingredients in Posilac?

Public discussions and regulatory reviews have acknowledged concerns related to the product's use. These included the potential for increased IGF-1 levels in milk and the potential for antibiotic residues due to the reported increase in mastitis risk associated with the drug.


Q: What happens if Posilac is accidentally used more frequently than advised?

The administration protocol establishes a fixed 14-day interval between injections. Regulatory documents define this fixed frequency but do not provide specific instructions or procedures regarding how to handle a case of accidental over-frequency.


Q: What does the packaging typically include besides the Posilac product itself?

According to trade and classification documents, the packaging is designed for professional use. It typically includes not only the Posilac syringes but also needles and instructions for use by the dairymen administering the product.


Q: Is Posilac suitable for individuals following a vegetarian or vegan diet?

The active ingredient is a synthetic protein analog of a bovine hormone, produced using recombinant DNA technology. The product is also formulated with a sesame oil and aluminum monostearate excipient. The official documentation is factual and does not provide guidance on its suitability for specific human diets like vegetarian or vegan.

How should Posilac be stored and disposed of?

How to Store and Dispose of Posilac?

The official requirements for storing and disposing of Posilac (sometribove) are strictly defined in regulatory labeling to maintain product stability and ensure safe handling.

Storage Requirements

Condition Requirement
Temperature Mandatory refrigeration between 2 C and 8 C (36 F to 46 F).
Prohibition DO NOT FREEZE. Avoid prolonged exposure to direct sunlight or excessively high temperature.
Handling Allow syringes to warm to room temperature (15 C to 30 C) before use.
Safety KEEP OUT OF REACH OF CHILDREN is a mandated warning.

Disposal Instructions

Disposal of Posilac used syringes must adhere to regulated sharps protocols. Used syringes are to be placed in a leak-resistant, puncture-resistant container. This disposal method must be carried out in accordance with all applicable Federal, state, and local regulations concerning pharmaceutical and sharps waste.

Attention! Always consult to a doctor or pharmacist before using pills or medicines.

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