Does calcium interact with any medications?

Calcium carbonate can interfere with levothyroxine absorption, and the FDA label says not to take them within 4 hours of each other.

What this means for you

Calcium carbonate and levothyroxine do not belong in the same hour. The FDA label says to keep them at least four hours apart, because the calcium interferes with absorbing the thyroid hormone.

This catches people out because calcium carbonate is not only sold as a calcium supplement. It is also the active ingredient in many antacids, so someone can be taking it without thinking of it as a supplement at all.

The fix is timing rather than avoidance. Thyroid tablet first thing on an empty stomach, calcium with a later meal. That order also happens to suit calcium itself, which is absorbed better in doses of 500 mg or less taken with food.

Three other pairings are worth keeping apart by a few hours. Taking calcium supplements at the same time as a quinolone antibiotic such as ciprofloxacin or moxifloxacin reduces how much of the antibiotic is absorbed, so a course of one is a reason to move the supplement to the other end of the day. It does the same to iron tablets, and unusually it works on both the iron in meat and the iron in plants, which is why the two supplements should not share a glass of water. And thiazide diuretics push blood calcium up rather than down, so anyone on one who also takes a large calcium supplement should have that checked rather than assumed.

None of this means giving anything up. It means putting a few hours between two tablets that were never meant to arrive together.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

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