Calcium supplements can interfere with the absorption of which other supplement or drug?

• A. Vitamin C
• B. Iron (and tetracycline antibiotics)
• C. Magnesium
• D. Zinc

Answer: (B)

Calcium can bind with certain substances in the gut and form complexes that aren’t readily absorbed. The most clinically important example is iron. Calcium forms insoluble complexes with iron, which reduces iron absorption and can blunt the effectiveness of iron therapy or worsen iron deficiency. It’s also true that calcium can chelate tetracycline antibiotics, lowering their absorption and effectiveness, so these should be spaced apart from calcium.

The other options aren’t the best fit because vitamin C actually helps iron absorption, so calcium interference isn’t the concern there; magnesium and zinc can be affected by mineral competition in some cases, but the strongest, most well-known interaction involving calcium is with iron (and tetracyclines), which is why that answer is correct.