Multivitamins and Medicine Interactions: Warfarin, Thyroid Medicine, Antibiotics and More

If you take a prescribed medicine, ask first

Multivitamins are often assumed to be harmless add-ons, but several ingredients are named on official medicine pages as ones that may not mix well with common prescriptions. NCCIH says supplements may interact with medicines. [1] Tell your doctor or pharmacist about every vitamin and supplement you use, and do not stop, skip or change a prescribed medicine on your own. If you take warfarin and notice signs of serious bleeding, such as blood in urine or stool, coughing or vomiting blood, or a sudden severe headache with confusion or weakness, the NHS lists these as symptoms of serious bleeding and tells UK readers to get urgent help; in the US, call 911. [2]

Evidence labels used here: established interaction (named on an official medicine page), mixed, insufficient evidence.

What the sources say, medicine by medicine

1. Warfarin (blood thinner)

   Possible

   The NHS says not to take vitamin K supplements while on warfarin, and that foods high in vitamin K can affect how it works. [2] MedlinePlus says vitamin K can affect how warfarin works and that blood vitamin K should be kept constant day to day. [3]

   Multivitamin ingredient involved: Vitamin K

2. Levothyroxine (thyroid hormone)

   Possible

   The NHS lists calcium and iron supplements as things that may not mix well, and says biotin can affect thyroid tests. [4]

   Multivitamin ingredient involved: Calcium, iron; biotin

3. Ciprofloxacin (antibiotic)

   Possible

   The NHS advises a gap of at least 2 hours from calcium-fortified drinks and dairy. [5] In 12 healthy men, ferrous sulfate and a zinc-containing multivitamin both lowered ciprofloxacin absorption. [6]

   Multivitamin ingredient involved: Calcium; iron; zinc

4. Doxycycline (antibiotic)

   Possible

   The NHS lists medicines or supplements containing iron or zinc as ones that may not mix well. [7]

   Multivitamin ingredient involved: Iron, zinc

Warfarin and vitamin K in a multivitamin

Most multivitamins contain far less vitamin K1 than is thought to affect warfarin, according to one trial's authors, but a controlled study found an effect in people who were low in vitamin K. In 16 warfarin-treated patients given a multivitamin tablet with 25 micrograms of vitamin K1 daily for 4 weeks, all 9 with low vitamin K status had a subtherapeutic INR (a clotting measure), compared with 1 of 7 with normal status. [8] Small numbers, one product type. The lesson is that stability matters: switching to or from a vitamin-K-containing product can change a clotting test, and a clinic can only adjust if it knows. Ask the warfarin clinic or pharmacist before any change. See Vitamin K.

Thyroid medicine, calcium, iron and biotin

In a JAMA study of 20 people on stable levothyroxine, 1,200 mg a day of elemental calcium (as carbonate) taken with it for 3 months lowered thyroxine and raised thyrotropin; 20% had thyrotropin above the normal range. [9] That is a study amount, not advice. Biotin can also change lab results: the FDA says biotin, often found in dietary supplements, can significantly interfere with certain lab tests and cause incorrect results. [10] Tell whoever orders your blood tests about biotin. See Biotin.

Antibiotics and minerals

Minerals in a multivitamin can lower the absorption of some antibiotics. The 1989 volunteer study found iron cut ciprofloxacin absorption sharply (area under the curve 14.5 to 5.4 micrograms.h/ml) and a multivitamin with zinc cut it less (11.3). [6] The authors said the zinc-containing multivitamin's responsible component needed more study. A pharmacist can say whether and how your antibiotic should be spaced from supplements.

Other medicines

The pages we read for this guide covered warfarin, levothyroxine, ciprofloxacin and doxycycline only. Whether a multivitamin interacts with other medicines (for example seizure, cancer or transplant medicines) is not described in the sources we read, so ask a pharmacist. See How interactions work and Supplements before surgery. Check Multivitamin timing for the absorption studies.

Pregnancy, children, kidney and liver conditions

Many supplements have not been tested in pregnant women, nursing mothers or children. [1] People with kidney or liver conditions, and older adults on many medicines, should have a clinician or pharmacist check any product. Product quality varies; see Quality testing and certifications.