Zinc side effects

At typical supplement doses (8-40 mg/day), the most common zinc side effects are nausea, a metallic or bitter taste, and stomach upset, especially taken without food. At the much higher doses used short-term for cold treatment (75 mg/day or more), nausea and bad taste were significantly more common than placebo in clinical trials. Taken well above the daily upper limit for weeks or months, zinc can also reduce copper status.

Common, dose-related side effects

NIH's Office of Dietary Supplements lists nausea, dizziness, headache, gastric distress, vomiting and loss of appetite as signs of too much zinc. A 2013 Cochrane review of zinc lozenge trials for the common cold — dosed at 75 mg/day or more — found overall adverse events, bad taste and nausea all significantly more common in the zinc groups than placebo (bad taste roughly twice as likely, nausea about twice as likely). These are lozenge-specific, high-dose findings; they don't necessarily describe what happens at a typical 8-40 mg/day capsule dose, though the same categories of GI upset are the ones to watch for at any dose.

Copper status at higher, sustained doses

The concern that matters most for long-term use isn't acute nausea — it's copper. A 1984 controlled study gave healthy men 50 mg/day of zinc for six weeks and found a significant drop in the activity of a copper-dependent enzyme, a sign that zinc was already interfering with copper status at that dose and duration. This is the reasoning behind the 40 mg/day adult upper intake level: it's set well below doses shown to affect copper handling over time. See who should not take zinc and zinc dosage for more.

Other effects noted at high, chronic intakes

NIH's Office of Dietary Supplements also notes that very high, sustained zinc intake has been associated with reduced immune function — the opposite of the effect people are often taking zinc for in the first place. These findings describe chronic intake well above the RDA, not the ordinary range of a daily multivitamin-level dose.

Questions people ask

What are the side effects of zinc?

At typical doses: nausea, a metallic taste, and stomach upset, especially without food. At high, sustained doses: reduced copper status and reduced immune function, according to NIH's Office of Dietary Supplements.

Can zinc supplements cause an allergic reaction?

An allergic reaction to any supplement ingredient, including a specific zinc compound or a capsule's other ingredients, is possible but not common. A rash, swelling or difficulty breathing after taking any supplement warrants stopping it and contacting a doctor.

Do zinc side effects get worse with higher doses?

Generally yes — the trial data above shows nausea and bad taste rising at the 75 mg/day and higher doses used for cold treatment, and copper effects showing up at 50 mg/day sustained over weeks. Staying within the 40 mg/day adult upper limit for daily use avoids the doses where these effects were documented.

Does zinc cause diarrhea?

Diarrhea isn't among the most commonly reported zinc side effects in the sources behind this page; nausea, taste changes and stomach upset are more typical. Any persistent GI symptom after starting a supplement is worth mentioning to a doctor.

Related reading: how much zinc should you take a day, who should not take zinc, and zinc and medications: what to check.