Which supplements shouldn't be taken together? A simple timing guide

Most supplements can be taken together without a second thought. A handful can't. Minerals in particular compete for absorption, and a few everyday habits, like washing down your iron with coffee, can cut what you absorb by half or more. Here are the combinations worth keeping apart, with what the research shows.

Iron is the pickiest supplement

Calcium blocks iron. In absorption studies, 300 to 600 mg of calcium cut iron absorption by 50 to 60%, and even 165 mg, about the calcium in a glass of milk, had a similar effect. If you take iron, keep it a couple of hours away from calcium supplements and dairy.

Coffee and tea block it too. A cup of coffee with a meal reduced iron absorption by 39%, and tea by 64%. Timing matters: coffee drunk an hour before the meal had no effect, but coffee an hour after reduced absorption just as much as drinking it with the meal. Take iron with water, and have your coffee first or save it for later in the day.

Vitamin C is optional. It's long been recommended alongside iron, but in a randomized trial of 440 adults with iron-deficiency anemia, iron alone raised hemoglobin just as well as iron plus vitamin C.

Less often can mean more. In iron-depleted women, a single dose every other day was absorbed more efficiently than daily doses, and splitting the dose in two didn't help. Iron dosing should be based on your blood levels, so work out the schedule with your clinician.

Zinc, iron, and copper

Zinc and iron compete when taken together as supplements: iron at two to three times the zinc dose substantially reduced zinc absorption. Take them at different times of day. Zinc also affects copper: high doses taken for long periods can cause copper deficiency, which is easy to miss. Stay within the 40 mg a day adult upper limit for zinc unless a clinician is supervising.

Calcium works best in small doses

Your body absorbs calcium best in portions of about 500 mg or less, so if you take more than that, split it across the day, ideally with meals. Meeting most of your calcium needs from food and supplementing only the gap is the safest approach.

Supplements and medications

• Thyroid medicine (levothyroxine): calcium carbonate, citrate, and acetate each cut levothyroxine absorption by about 20 to 25% when taken together. Iron and magnesium can interfere too. Take thyroid medicine on its own and keep minerals well apart; your pharmacist can give you an exact gap.
• Certain antibiotics and osteoporosis medicines: calcium, magnesium, iron, and zinc can bind them in the gut. Follow the spacing on the prescription label.
• Blood thinners: vitamin K directly affects warfarin, and several supplements can add to bleeding risk. Talk to your prescriber before adding anything. Our guide to vitamin D and K2 explains the vitamin K issue.
• St. John's wort interacts with many prescription drugs, including birth control and antidepressants. See our St. John's wort page before taking it.

What to take with food

• Fat-soluble vitamins (A, D, E, and K) absorb better with a meal that contains some fat. In one study, vitamin D was absorbed 32% better with a fat-containing meal than with a fat-free one.
• Magnesium, fish oil, and most multivitamins are easier on the stomach with food.
• Iron absorbs best on an empty stomach, but if it upsets yours, a small meal without dairy, coffee, or tea is a reasonable compromise.
• Creatine can be taken any time, with or without food. See our guide to when to take creatine.

One way to arrange a day

If you take several of these, a simple pattern keeps the conflicts apart: iron first thing with water (coffee before it, or a couple of hours after); vitamin D, a multivitamin, and fish oil with your largest meal; calcium split between meals, away from iron; zinc at a different meal from iron; and magnesium in the evening. Adjust it around any medications with your pharmacist.

The short version

Keep iron away from calcium, dairy, coffee, and tea; keep zinc away from iron; split calcium into small doses; take fat-soluble vitamins with a meal; and keep all minerals well away from thyroid medicine and certain antibiotics. When in doubt, a pharmacist can check your full list in a few minutes.