N-acetylcysteine (NAC) is a derivative of the amino acid cysteine. This page reviews oral supplementation studies for training-related goals.
There is currently no convincing advantage of oral NAC for strength, power or recovery after regular training. Small trials report differing early and late recovery results. [1] [2] [3] [4]
| Goal or question | Result | Confidence | Studies | Population studied | Key sources |
|---|---|---|---|---|---|
| Power during high-intensity cycling | This trial provides no basis for choosing NAC to increase power.Mean power: 401 versus 400 W in the first test and 400 versus 401 W in the second. There was no advantage in this protocol. | 2/5 | 1One direct trial described; the count refers to the named publication, not the entire literature.N-Acetyl cysteine does not improve repeated intense endurance cycling performance of well-trained cyclists ↗ | 11 well-trained cyclists | |
| Recovery of strength after exercise | There is no convincing advantage for routine recovery after gym sessions.Strength declined less with NAC during the first two days; by day eight, the authors reported complete recovery only with placebo. The early result did not translate into faster complete recovery. | 1/5 | 1One direct trial described; the count refers to the named publication, not the entire literature.Thiol-based antioxidant supplementation alters human skeletal muscle signaling and attenuates its inflammatory response and recovery after intense eccentric exercise ↗ | 10 men after damaging eccentric exercise | |
| Isometric strength during repeated contractions | This experiment does not support choosing NAC to increase strength or muscle mass.Maximum isometric strength did not differ significantly between groups. A later decline in strength relative to one group's own baseline does not establish an advantage over placebo. | 2/5 | 1One direct trial described; the count refers to the named publication, not the entire literature.Acute Effect of Oral N-Acetylcysteine Supplementation on Fatigue Effect and Isometric Force Production in Physically Active People ↗ | 23 physically active people: 12 NAC and 11 placebo | |
| Soreness after exercise | There is currently no reliable advantage for a regular regimen to prevent soreness.Soreness was lower on days 1–4 but higher on days 5–6 after the second exercise session. Results in opposite directions do not confirm sustained pain relief. | 1/5 | 1One direct trial described; the count refers to the named publication, not the entire literature.Acute Effect of Oral N-Acetylcysteine on Muscle Soreness and Exercise Performance in Semi-Elite Rugby Players ↗ | 17 male rugby players |
The study count refers to the cited source. Reviews with overlapping trials and outcomes from one study are not added together.
This trial provides no basis for choosing NAC to increase power.
Study findings. Mean power: 401 versus 400 W in the first test and 400 versus 401 W in the second. There was no advantage in this protocol.
The confidence scale assesses the reliability of this conclusion, not the probability of an individual result.
There is no convincing advantage for routine recovery after gym sessions.
Study findings. Strength declined less with NAC during the first two days; by day eight, the authors reported complete recovery only with placebo. The early result did not translate into faster complete recovery.
The confidence scale assesses the reliability of this conclusion, not the probability of an individual result.
This experiment does not support choosing NAC to increase strength or muscle mass.
Study findings. Maximum isometric strength did not differ significantly between groups. A later decline in strength relative to one group's own baseline does not establish an advantage over placebo.
The confidence scale assesses the reliability of this conclusion, not the probability of an individual result.
There is currently no reliable advantage for a regular regimen to prevent soreness.
Study findings. Soreness was lower on days 1–4 but higher on days 5–6 after the second exercise session. Results in opposite directions do not confirm sustained pain relief.
The confidence scale assesses the reliability of this conclusion, not the probability of an individual result.
The rating considers study quality, directness and applicability to the stated group and regimen. It is an editorial scale, not the probability of an individual result.
Oral NAC and the intravenous medicine differ. Nausea, diarrhea and abdominal pain may occur. Blood pressure lowering may increase with nitroglycerin or isosorbide. Combinations with antibiotics and other medicines require checking the product instructions and consulting a professional; the Canadian monograph excludes concurrent use with antibiotics and nitroglycerin. For the product category considered, the Canadian monograph limits the amount to 600 mg/day in adults and excludes concurrent use with antibiotics and nitroglycerin. The RIVM assessment of up to 1200 mg/day belongs to a different safety assessment system and does not replace the instructions for a specific product. The higher amounts studied in RCTs are not regimens for self-administration.
Dietary Supplements for Immune Function and Infectious Diseases — Health Professional Fact Sheet (NIH ODS) ↗ · N-Acetyl-L-Cysteine — Natural Health Product Monograph (Health Canada) ↗ · Risk assessment of food supplements containing N-acetylcysteine (NAC) — RIVM letter report 2024-0086 ↗
Oral NAC: The form covered by the supplement studies; check the amount of NAC itself on the packaging. Intravenous or inhaled NAC: Different routes of administration; their regimens cannot be applied to capsules. L-cysteine, cystine, glutathione: Different substances; their amounts and effects should not be added to NAC as a single dose.
Protein foods, including meat, fish, eggs, dairy products, soy and legumes, supply amino acids including cysteine. Cysteine in protein and N-acetylcysteine in a capsule are different forms; a serving of protein food cannot be converted into a NAC dose.
| Publication | What was studied | Participants | Form and dose | Result |
|---|---|---|---|---|
| N-Acetyl cysteine does not improve repeated intense endurance cycling performance of well-trained cyclists ↗ | Power during high-intensity cycling | 11 well-trained cyclists | Oral NAC: 20 mg/kg as a single dose. A single dose; two four-minute tests | Mean power: 401 versus 400 W in the first test and 400 versus 401 W in the second. There was no advantage in this protocol. |
| Thiol-based antioxidant supplementation alters human skeletal muscle signaling and attenuates its inflammatory response and recovery after intense eccentric exercise ↗ | Recovery of strength after exercise | 10 men after damaging eccentric exercise | Oral NAC: 20 mg/kg per day after exercise. Eight days of follow-up after exercise | Strength declined less with NAC during the first two days; by day eight, the authors reported complete recovery only with placebo. The early result did not translate into faster complete recovery. |
| Acute Effect of Oral N-Acetylcysteine Supplementation on Fatigue Effect and Isometric Force Production in Physically Active People ↗ | Isometric strength during repeated contractions | 23 physically active people: 12 NAC and 11 placebo | Oral NAC: 2.4 g/day. Eight days | Maximum isometric strength did not differ significantly between groups. A later decline in strength relative to one group's own baseline does not establish an advantage over placebo. |
| Acute Effect of Oral N-Acetylcysteine on Muscle Soreness and Exercise Performance in Semi-Elite Rugby Players ↗ | Soreness after exercise | 17 male rugby players | Oral NAC: 1 g/day. Six days | Soreness was lower on days 1–4 but higher on days 5–6 after the second exercise session. Results in opposite directions do not confirm sustained pain relief. |
