Zinc

Is zinc safe?

At the amounts in a multivitamin, yes. Zinc is an essential mineral and adults need 8 to 11 mg a day, most of which comes from meat, shellfish, dairy and whole grains. The trouble is that the zinc aisle is full of products well above that. Tablets of 50 mg are a standard size, and a pack of cold lozenges taken as directed can deliver more than 100 mg a day. The US upper limit is 40 mg a day from all sources, and EFSA's is lower at 25 mg.

Zinc's one real hazard at those doses is slow and easy to miss: it starves the body of copper.

How zinc causes copper deficiency

High zinc intake makes gut cells produce metallothionein, a protein that grabs copper and holds it inside the cell until the cell is shed. The copper never reaches the blood. Doctors use this effect deliberately to treat Wilson's disease, a condition of copper overload. In everyone else it is a side effect.

According to the Linus Pauling Institute, signs of copper deficiency have appeared at total zinc intakes of 60 mg a day (50 mg from a supplement plus 10 mg from food) within 10 weeks. That is one ordinary 50 mg tablet a day. Cold lozenges taken for six to eight weeks are likely to have the same effect.

Copper deficiency develops quietly. It causes anaemia and low white blood cell counts that can look like a bone marrow disorder, and in some people damage to the spinal cord, with numbness in the feet, poor balance and an unsteady walk. Nerve damage does not always reverse once copper is replaced.

An unusual source showed how real this is. In 2008, neurologists described four patients with low copper, high zinc and neurological disease in the journal Neurology. All wore dentures and used very large amounts of denture cream, which contained 17,000 to 34,000 micrograms of zinc per gram. Heavy users of zinc-containing denture adhesive, at two tubes a week or more, are now a recognised risk group.

Zinc nasal gels and loss of smell

The most serious zinc safety action was about nasal products, not tablets. By 2009 the FDA had received more than 130 reports of anosmia, the loss of the sense of smell, linked to Zicam intranasal zinc products, and it told consumers to stop using the Cold Remedy Nasal Gel and two swab products. The agency's position, restated in its 2022 guidance on homeopathic drugs, was that using them as labelled exposed the nose to significant daily doses of zinc.

Zinc applied to the nose damages smell receptors in animal studies, and the evidence that nasal zinc shortens colds is weak. The Linus Pauling Institute's advice is simply to avoid zinc nasal gels and sprays, since the loss may be permanent. Losing smell is more than an inconvenience: you may not notice smoke, a gas leak or spoiled food.

Short-term high doses

Large single doses irritate the stomach. Doses of 225 to 450 mg usually cause vomiting, and milder nausea and stomach pain show up at 50 to 150 mg a day. Taking zinc with food helps. Trials of cold lozenges at 50 to 180 mg a day for one to two weeks found bad taste and nausea, but no serious harm. The risk comes from turning a cold remedy into a daily habit.

Interactions with medicines

Medicine	What happens	What to do
Quinolone antibiotics (ciprofloxacin) and tetracyclines (doxycycline)	Zinc and antibiotic both absorbed less	Separate by at least 2 hours
Cephalexin	Less antibiotic absorbed	Separate doses
Penicillamine	Less drug absorbed; the drug also depletes zinc	Ask your doctor
Atazanavir, ritonavir (HIV)	Less drug absorbed	Ask your HIV clinic
Thiazide diuretics, valproate	Raise zinc loss	Your doctor may check levels
Diabetes medicines	Zinc may lower blood sugar slightly	Use with caution

Zinc also competes with iron for absorption, so high-dose iron and zinc are best taken at different times.

Product quality

Zinc gluconate, citrate, picolinate and oxide are cheap, stable salts, and independent testers rarely report contamination. Labels can confuse, though: a tablet sold as "zinc gluconate 50 mg" may contain only about 7 mg of elemental zinc, while "zinc 50 mg" means 50 mg of the mineral itself. The limit refers to elemental zinc. USP VerifiedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary and NSF marks are common on zinc products.

Who should be careful

• Anyone taking a zinc supplement above 25 to 40 mg a day for more than a few weeks. Ask your doctor about checking copper.
• People who use a lot of denture adhesive. Check the label for zinc.
• People with unexplained anaemia, low white cell counts or numb feet who take zinc.
• Anyone on the medicines in the table above. Talk to your doctor or pharmacist about timing.

Magnesium and vitamin C, often sold alongside zinc for immunity, have wider safety margins.

How we scored it

Zinc scores 77/100. Product quality (84) is its strongest factor. Dose safety (72) is its weakest because 50 mg tablets and long lozenge courses sit above the upper limit, and organ toxicity (74) reflects copper-deficiency nerve damage and the loss of smell from nasal products. Interactions and regulatory record score 78 each.