Zinc is an essential trace mineral. Nearly 100 enzymes need it to work, it holds together the “zinc finger” proteins that switch genes on and off, and it is needed for growth, immune function and reproduction. Red meat, some seafood and whole grains are the main food sources. It is also sold in many supplement forms, in cold lozenges, and in two FDA-approved drugs.
In fitness circles, zinc is promoted as a testosterone booster, often as part of “ZMA” products. The evidence splits cleanly by zinc status. In men made zinc deficient, testosterone falls, and in deficient older men it rises with zinc. In men who already get enough zinc, controlled trials show no testosterone gain. The most important safety fact: the US upper limit is 40 mg/day from all sources, set because higher intakes lower copper status, and long-term high doses can cause copper deficiency with anemia and nerve damage.
This page summarizes what the National Academies’ dietary reference report, FDA labels and clinical trials report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Type | Essential trace mineral; dietary supplement and FDA-approved drug ingredient |
| Body distribution | Over 85% in skeletal muscle and bone; plasma holds only about 0.1% |
| Adult RDA (US) | Men 11 mg/day; women 8 mg/day (pregnancy 11 mg/day, lactation 12 mg/day, ages 19+) |
| Upper limit (US) | 40 mg/day for adults, from food, water and supplements combined |
| Typical US intake | Median about 14 mg/day in men and 9 mg/day in women aged 19–50 |
| Doses in cited trials | 30 mg/day (testosterone and fertility trials); 50 mg/day (copper studies); 80 mg/day (AREDS eye trial) |
| Half-life | No single plasma value; in a whole-body tracer study, the slow turnover phase had a half-life of about 218 days in reference subjects |
| Approved drugs | Zinc acetate (Galzin) for Wilson disease; zinc sulfate injection for intravenous nutrition |
| WADA status | Not on the 2026 Prohibited List |
What it is
The National Academies’ Institute of Medicine (IOM) groups zinc’s roles into three types (IOM 2001):
- Catalytic. Nearly 100 specific enzymes depend on zinc, including RNA polymerases, alcohol dehydrogenase, carbonic anhydrase and alkaline phosphatase.
- Structural. Zinc folds “zinc finger” proteins, many of which are gene-regulating transcription factors, such as the retinoic acid and vitamin D receptors. It also stabilizes copper-zinc superoxide dismutase, an antioxidant enzyme.
- Regulatory. Zinc switches on genes such as metallothionein, a protein that binds and shuttles zinc (and copper) inside cells.
Over 85% of body zinc sits in skeletal muscle and bone. Plasma zinc is only about 0.1% of the total and is held tightly at about 10–15 μmol/L (≈65–98 μg/dL). Infection, trauma and stress lower it, and meals lower it too, so plasma zinc is an insensitive marker of zinc status. The IOM cites a pre-breakfast cut-off of 70 μg/dL for low values (IOM 2001).
Dietary reference intakes
The IOM set the Recommended Dietary Allowance (RDA) with a factorial method: it added up daily zinc losses (urine, skin and sweat, semen or menses, and gut) and worked out the intake needed to absorb enough to replace them (IOM 2001).
| Group | EAR (mg/day) | RDA (mg/day) | UL (mg/day) |
|---|---|---|---|
| Children 1–3 y | 2.5 | 3 | 7 |
| Children 4–8 y | 4 | 5 | 12 |
| Ages 9–13 y | 7 | 8 | 23 |
| Boys 14–18 y | 8.5 | 11 | 34 |
| Girls 14–18 y | 7.3 | 9 | 34 |
| Men 19 y and older | 9.4 | 11 | 40 |
| Women 19 y and older | 6.8 | 8 | 40 |
| Pregnancy, 19–50 y | 9.5 | 11 | 40 |
| Lactation, 19–50 y | 10.4 | 12 | 40 |
EAR is the Estimated Average Requirement, the intake that meets the needs of half of healthy people. The RDA is set at 120% of the EAR.
The IOM flags two groups who may need more:
- Vegetarians. Grains and legumes are rich in phytate, which binds zinc in the gut. The requirement may be as much as 50% higher for strict vegetarians whose dietary phytate:zinc molar ratio exceeds 15:1.
- Heavy drinkers. Long-term alcohol use impairs zinc absorption and increases urinary loss. About 30–50% of people with alcoholism have low zinc status.
Supplement forms and zinc content
Every supplement delivers zinc as a salt or chelate, and the label dose is usually given as elemental zinc. Forms differ in how much zinc they carry by weight and, to a smaller degree, in how well that zinc is absorbed. In a randomized, double-masked crossover study, 15 healthy adults took 10 mg of zinc from each of three forms without food, and absorption was measured with stable isotopes (Wegmüller et al., J Nutr 2014):
| Form | Zinc content by weight | Fractional absorption (10 mg, fasting) | Notes |
|---|---|---|---|
| Zinc sulfate | 23% | About 71% in an earlier study using the same method | Water soluble; cheap; metallic taste |
| Zinc acetate | 30% | Not measured in this study | Form in Galzin; also used in lozenges |
| Zinc gluconate | 14% | 60.9% (IQR 50.6–71.7) | Most common lozenge form |
| Zinc citrate | 31% | 61.3% (IQR 56.6–71.0) | Slightly water soluble |
| Zinc oxide | 80% | 49.9% (IQR 40.9–57.7) | Insoluble; 3 of 15 people absorbed little or none |
| Zinc bisglycinate | Not reported in cited sources | Bioavailability 43.4% higher than gluconate (single 15 mg dose, 12 women) | Data from one small study (Gandia et al., 2007) |
The zinc-content figures for sulfate, acetate, gluconate, citrate and oxide come from the Wegmüller paper. The authors noted that with food, absorption from supplements and fortified foods falls to about 10–25%, and the gap between zinc oxide and soluble forms shrinks.
How it works in the body
Most zinc is absorbed in the small intestine, mainly the jejunum, by a transporter-driven process that saturates at higher doses. Absorption becomes more efficient when the body is low in zinc (IOM 2001). Newly absorbed zinc travels to the liver in the portal blood, mostly bound to albumin. The zinc sulfate injection label states that about 80% of serum zinc is bound to albumin, with the rest bound to alpha-2-macroglobulin and amino acids (zinc sulfate injection label).
The gut, not the kidney, sets zinc balance. Pancreatic and intestinal secretions put zinc back into the gut. Fecal losses range from under 1 mg/day on a zinc-poor diet to over 5 mg/day on a zinc-rich one. Urinary losses are under 10% of fecal losses (IOM 2001).
Things that lower zinc absorption, per the IOM:
- Phytate in grains and legumes, which binds zinc and calcium together. Leavening bread with yeast reduces phytate.
- Iron supplements taken without food. At an iron:zinc ratio of 3:1 in water, zinc absorption fell; with a meal or with heme iron, no effect was seen.
- Malabsorption from Crohn’s disease, celiac disease, sprue or short bowel syndrome.
Animal protein improves zinc absorption compared with plant protein.
There is no meaningful plasma half-life for a supplement dose, because the body pools and recycles zinc. Whole-body counting after a radioactive zinc tracer gives a sense of the turnover. In one such study, the slow phase of tracer loss had a half-life of 218 ± 27 days in the lab’s reference values (Crofton et al., Am J Clin Nutr 1990).
Approved medical uses
Zinc appears in two FDA-labeled prescription products:
| Product | Indication | Labeled adult dose |
|---|---|---|
| Galzin (zinc acetate capsules, 25 mg and 50 mg zinc) | Maintenance treatment of Wilson disease (a genetic copper overload disorder) after initial chelation therapy | 50 mg zinc three times daily, on an empty stomach |
| Zinc sulfate injection (1 mg/mL and 5 mg/mL zinc) | Source of zinc for intravenous (parenteral) nutrition | 3 mg/day for metabolically stable adults |
Sources: Galzin label; zinc sulfate injection label.
Galzin works because high zinc doses induce metallothionein in gut cells. Metallothionein traps dietary and secreted copper, which is then shed in the stool. This is the same mechanism behind the copper deficiency that high-dose zinc supplements can cause (see below).
The IOM states that its upper limit does not apply to people taking zinc under medical supervision (IOM 2001).
What the research shows
Testosterone
The testosterone claim rests on a real biological link, but the benefit depends on starting zinc status.
Zinc deficiency lowers testosterone. The IOM lists delayed sexual maturation and impotence among the features of severe zinc deficiency (IOM 2001). The key human study combined three parts (Prasad et al., Nutrition 1996):
| Part | Design and n | Result |
|---|---|---|
| Cross-sectional | 40 healthy men, aged 20–80 | Cell zinc correlated with serum testosterone (lymphocyte zinc r = 0.43, p = 0.006) |
| Zinc restriction | 4 healthy young men, zinc-restricted diet for 20 weeks | Testosterone fell from 39.9 to 10.6 nmol/L (≈1,151 to 306 ng/dL), p = 0.005 |
| Zinc supplementation | 9 marginally zinc-deficient older men (mean age 64), 459 μmol/day zinc (≈30 mg) as zinc gluconate for 3–6 months | Testosterone rose from 8.3 to 16.0 nmol/L (≈239 to 461 ng/dL), p = 0.02 |
These groups were very small and uncontrolled, but the direction is consistent. In a later cross-sectional study of 720 men with sexual complaints, testosterone fell as serum zinc fell, even after adjusting for cortisol (Miyoshi et al., Int J Urol 2023). A 2023 systematic review of 8 clinical and 30 animal studies concluded that zinc deficiency lowers testosterone and that the effect of supplements depends on baseline zinc and testosterone (Te et al., J Trace Elem Med Biol 2023). Most of that review’s evidence came from animals.
In zinc-replete men, controlled trials show no testosterone gain:
- ZMA in trained men. 42 resistance-trained men were randomized to a ZMA-type product (30 mg zinc, 450 mg magnesium and vitamin B6 daily, plus a mucuna extract) or placebo for 8 weeks of training. Total and free testosterone, IGF-1, body composition and strength did not differ from placebo. Total testosterone changed by −0.47 ng/mL on ZMA vs −0.12 ng/mL on placebo (p = 0.50) (Wilborn et al., JISSN 2004).
- ZMA in men with adequate intake. In 14 regularly exercising men whose baseline zinc intake was 11.9–23.2 mg/day, ZMA raised serum zinc but did not change serum total or free testosterone or the urinary pattern of testosterone metabolites (Koehler et al., Eur J Clin Nutr 2009).
- Folate plus zinc in subfertile men. A meta-analysis of randomized trials found no effect of folate plus zinc on serum testosterone vs placebo (p = 0.86) (Irani et al., Urol J 2017).
Taken together, zinc corrects low testosterone caused by zinc deficiency. It does not raise testosterone above normal in men who already get enough. For actual testosterone deficiency, see Testosterone Cypionate.
Male fertility
The largest trial is the Folic Acid and Zinc Supplementation Trial (FAZST). It randomized 2,370 men from couples seeking infertility treatment to 5 mg folic acid plus 30 mg elemental zinc daily, or placebo, for 6 months (Schisterman et al., JAMA 2020):
| Outcome | Folic acid + zinc | Placebo | Difference |
|---|---|---|---|
| Live birth | 34% (404) | 35% (416) | −0.9% (95% CI −4.7% to 2.8%) |
| Sperm DNA fragmentation | 29.7% | 27.2% | +2.4% (95% CI 0.5% to 4.4%) |
| Abdominal discomfort or pain | 6% | 3% | — |
| Nausea | 4% | 2% | — |
| Vomiting | 3% | 1% | — |
Sperm concentration, motility, morphology, volume and total motile count did not differ. The authors concluded that the findings do not support folic acid and zinc for male infertility.
Common cold
A 2024 Cochrane review pooled 34 randomized trials (8,526 participants) of zinc vs placebo, most using lozenges (Nault et al., Cochrane 2024):
| Use | Outcome | Effect (95% CI) | Certainty |
|---|---|---|---|
| Prevention | Risk of getting a cold | RR 0.93 (0.85–1.01); 9 trials, 1,449 people | Low |
| Treatment | Cold duration | 2.37 days shorter (0.53–4.21); 8 trials, 972 people | Low (very high heterogeneity) |
| Treatment | Non-serious side effects | RR 1.34 (1.15–1.55); 16 trials, 2,084 people | Moderate |
Zinc gluconate lozenge trials used doses of 45–276 mg/day (as reported for zinc gluconate) for 4.5–21 days. The review rated the evidence low certainty because doses, forms and outcomes varied widely.
Age-related macular degeneration
The Age-Related Eye Disease Study (AREDS) randomized 3,640 people aged 55–80 to antioxidants, zinc (80 mg as zinc oxide, with 2 mg copper as cupric oxide), both, or placebo, for an average of 6.3 years (AREDS Research Group, Arch Ophthalmol 2001). Antioxidants plus zinc reduced the odds of developing advanced macular degeneration (OR 0.72, 99% CI 0.52–0.98). The trial added the copper to prevent potential anemia from the high zinc dose.
Side effects and risks
Copper deficiency
This is the main long-term risk, and it is the basis of the 40 mg/day upper limit. High zinc intake induces metallothionein in gut cells, which traps copper and blocks its absorption (Galzin label).
The IOM based the UL on studies showing lower activity of erythrocyte copper-zinc superoxide dismutase (ESOD), a sensitive marker of copper status (IOM 2001):
| Study | Subjects | Zinc dose | Duration | Copper finding |
|---|---|---|---|---|
| Fischer et al., 1984 | 26 healthy men | 25 mg twice daily vs placebo | 6 weeks | ESOD activity fell, significant at week 6; plasma copper and ceruloplasmin unchanged |
| Yadrick et al., 1989 | 18 healthy women | 50 mg/day as zinc gluconate | 10 weeks | ESOD activity, serum ferritin and hematocrit fell |
The IOM took about 60 mg/day of total zinc (the 50 mg supplement plus about 10 mg from food) as the lowest dose with an adverse effect. It divided this by an uncertainty factor of 1.5 to set the adult UL of 40 mg/day (IOM 2001).
Over months to years, copper deficiency can become serious. The Galzin label lists post-marketing cases with these complications (Galzin label):
- anemia;
- low white cell counts (leukopenia, neutropenia, granulocytopenia);
- low platelets, or low counts of all blood cells (pancytopenia);
- myeloneuropathy, a spinal cord and nerve disorder that causes numbness, weakness and gait problems.
The zinc sulfate injection label describes the same picture, plus nephrotic-range proteinuria, after high supplemental zinc (about 10 times its 3 mg/day dose) taken for months to years (zinc sulfate injection label). Both labels advise stopping zinc and checking copper status if signs appear.
Hidden sources matter too. Four patients with copper deficiency and neurologic disease had used very large amounts of zinc-containing denture cream for years. The creams contained about 17,000–34,000 μg of zinc per gram (Nations et al., Neurology 2008).
Loss of smell from intranasal zinc
On June 16, 2009, the FDA told consumers to stop using three Zicam intranasal zinc cold products (nasal gel and nasal swabs). FDA had received more than 130 reports of loss of the sense of smell (anosmia), which may be long-lasting or permanent. Many people said it occurred with the first dose. FDA also sent the maker a warning letter stating that the products could not be marketed without FDA approval (FDA, 2009).
A case series of 17 patients described the typical pattern: sniffing the zinc gluconate gel deeply, an immediate burning sensation, and loss of smell within 48 hours. Seven of the 17 never developed cold symptoms (Alexander and Davidson, Laryngoscope 2006). This risk applies to zinc sprayed or swabbed into the nose, not to oral zinc.
Stomach upset and other effects of high doses
- Gastrointestinal effects. Nausea, vomiting, abdominal cramps, diarrhea, appetite loss and headache are the acute effects of excess zinc. Gastrointestinal distress has been reported at 50–150 mg/day of supplemental zinc (IOM 2001). In FAZST, folic acid plus 30 mg/day zinc roughly doubled nausea and abdominal pain rates vs placebo (see table above).
- HDL cholesterol. Some studies found lower HDL (“good”) cholesterol in men at 50–160 mg/day, but results were inconsistent, so the IOM did not use this to set the UL (IOM 2001).
- Gastric ulcers. The Galzin label lists post-marketing reports of gastric ulcers with long-term use.
- AREDS findings at 80 mg/day. Over about 6 years, the zinc arms had more hospitalizations for genitourinary causes (7.5% vs 4.9%; in men 8.6% vs 4.4%) and more self-reported anemia (13.2% vs 10.2%), although measured hematocrit did not differ (AREDS Research Group, Arch Ophthalmol 2001).
Interactions
- Quinolone antibiotics. The ciprofloxacin label says to take ciprofloxacin at least 2 hours before or 6 hours after products containing zinc, iron, calcium or magnesium, because these cations block its absorption (ciprofloxacin label).
- Iron. Iron supplements taken in water can reduce zinc absorption, and zinc can reduce iron absorption. Neither effect was seen when taken with a meal (IOM 2001).
- Copper. High-dose zinc lowers copper absorption (see above).
Who should be careful
- People with low copper intake or Menkes disease. The IOM notes that people with Menkes disease, a genetic copper transport defect, may be especially sensitive to excess zinc.
- Breastfeeding mothers on high-dose zinc therapy. The Galzin label notes zinc passes into breast milk and recommends women on zinc therapy not nurse, because of the risk of copper deficiency in the baby.
- Long-term intake above 40 mg/day outside medical supervision is where the documented copper risk lies.
Legal status and use in sport
In the US, zinc is sold as a dietary supplement and as an ingredient in prescription products such as Galzin and zinc sulfate injection. Intranasal zinc cold remedies were the subject of the 2009 FDA warning above.
Zinc is not listed on the WADA 2026 Prohibited List. Combination products should be judged by their full ingredient list. For example, the ZMA-type product in the Wilborn trial also contained a mucuna extract standardized for L-dopa. See Magnesium for the magnesium half of ZMA.
Key takeaways
- An essential mineral with a narrow comfort zone. Adult men need 11 mg/day and women 8 mg/day, and the US upper limit for all sources combined is 40 mg/day.
- Testosterone depends on zinc status. Zinc restriction lowered testosterone, and in a small uncontrolled study zinc raised it in deficient older men. In men who already get enough zinc, randomized trials of ZMA-type products found no testosterone, strength or body-composition gains.
- Copper is the long-term risk. Doses around 50 mg/day lowered a copper status marker within 6–10 weeks. Over months to years, high zinc can cause copper deficiency with anemia, low white cell counts and spinal cord and nerve damage.
- Intranasal zinc is linked to loss of smell. The loss may be permanent, and FDA warned consumers against three Zicam nasal products in 2009.
- Form changes the label math. Zinc oxide is 80% zinc by weight but was absorbed less well when fasting, while zinc gluconate is only 14% zinc. The RDA and UL refer to elemental zinc, not the weight of the salt.
Sources
- Institute of Medicine. 12 Zinc. In: Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academies Press; 2001. doi:10.17226/10026. NAP.
- Eton Pharmaceuticals. Galzin (zinc acetate) capsules, prescribing information. DailyMed.
- Apotex Corp. Zinc sulfate injection, prescribing information. DailyMed.
- Wegmüller R, Tay F, Zeder C, Brnic M, Hurrell RF. Zinc absorption by young adults from supplemental zinc citrate is comparable with that from zinc gluconate and higher than from zinc oxide. J Nutr. 2014;144(2):132–136. PubMed.
- Gandia P, Bour D, Maurette JM, et al. A bioavailability study comparing two oral formulations containing zinc (Zn bis-glycinate vs. Zn gluconate) after a single administration to twelve healthy female volunteers. Int J Vitam Nutr Res. 2007;77(4):243–248. PubMed.
- Crofton RW, Aggett PJ, Gvozdanovic S, et al. Zinc metabolism in celiac disease. Am J Clin Nutr. 1990;52(2):379–382. PubMed.
- Prasad AS, Mantzoros CS, Beck FW, et al. Zinc status and serum testosterone levels of healthy adults. Nutrition. 1996;12(5):344–348. PubMed.
- Miyoshi M, Tsujimura A, Miyoshi Y, et al. Low serum zinc concentration is associated with low serum testosterone but not erectile function. Int J Urol. 2023;30(4):395–400. PubMed.
- Te L, Liu J, Ma J, Wang S. Correlation between serum zinc and testosterone: a systematic review. J Trace Elem Med Biol. 2023;76:127124. PubMed.
- Wilborn CD, Kerksick CM, Campbell BI, et al. Effects of zinc magnesium aspartate (ZMA) supplementation on training adaptations and markers of anabolism and catabolism. J Int Soc Sports Nutr. 2004;1(2):12–20. PubMed.
- Koehler K, Parr MK, Geyer H, Mester J, Schänzer W. Serum testosterone and urinary excretion of steroid hormone metabolites after administration of a high-dose zinc supplement. Eur J Clin Nutr. 2009;63(1):65–70. PubMed.
- Irani M, Amirian M, Sadeghi R, Lez JL, Latifnejad Roudsari R. The effect of folate and folate plus zinc supplementation on endocrine parameters and sperm characteristics in sub-fertile men: a systematic review and meta-analysis. Urol J. 2017;14(5):4069–4078. PubMed.
- Schisterman EF, Sjaarda LA, Clemons T, et al. Effect of folic acid and zinc supplementation in men on semen quality and live birth among couples undergoing infertility treatment: a randomized clinical trial. JAMA. 2020;323(1):35–48. PubMed.
- Nault D, Machingo TA, Shipper AG, et al. Zinc for prevention and treatment of the common cold. Cochrane Database Syst Rev. 2024;5(5):CD014914. PubMed.
- Age-Related Eye Disease Study Research Group. A randomized, placebo-controlled, clinical trial of high-dose supplementation with vitamins C and E, beta carotene, and zinc for age-related macular degeneration and vision loss: AREDS report no. 8. Arch Ophthalmol. 2001;119(10):1417–1436. PubMed.
- Fischer PW, Giroux A, L’Abbé MR. Effect of zinc supplementation on copper status in adult man. Am J Clin Nutr. 1984;40(4):743–746. PubMed.
- Yadrick MK, Kenney MA, Winterfeldt EA. Iron, copper, and zinc status: response to supplementation with zinc or zinc and iron in adult females. Am J Clin Nutr. 1989;49(1):145–150. PubMed.
- Nations SP, Boyer PJ, Love LA, et al. Denture cream: an unusual source of excess zinc, leading to hypocupremia and neurologic disease. Neurology. 2008;71(9):639–643. PubMed.
- US Food and Drug Administration. FDA advises consumers not to use certain Zicam cold remedies: intranasal zinc product linked to loss of sense of smell. News release, June 16, 2009. FDA (archived).
- Alexander TH, Davidson TM. Intranasal zinc and anosmia: the zinc-induced anosmia syndrome. Laryngoscope. 2006;116(2):217–220. PubMed.
- Hikma Pharmaceuticals. Ciprofloxacin tablets, prescribing information. DailyMed.
- World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.