Use caution with

If you’re taking




To avoid decreased absorption of tetracycline, simply take calcium two hours before or after the antibiotic.

Coenzyme Q10


Not an absolute contraindication. If taking both, use caution and consider having a blood-clot test done to monitor any changes in prothrombin time.

Folic acid

The chemotherapy drug methotrexate and the epilepsy drugs phenobarbital, phenytoin, and primidone

Patients with acute lymphoblastic leukemia (ALL) should avoid folic acid in supplement form.

Garlic (Allium sativum)

Ticlopidine, warfarin

There is very little evidence to show that serious bleeding problems have developed from these combinations.


Carbidopa, ciprofloxacin, tetracycline, warfarin

An absorption issue; simply take iron two hours before or after the drug.

St. John’s wort (Hypericum perforatum)

Oral contraceptives; benzodiazepines; chemotherapy drugs; digoxin; antihistamines such as fexofenadine; antidepressants such as nefazodone, paroxetine, sertraline, trazodone, and tricyclic antidepressants; antiviral drugs such as atazanavir and indinavir; and warfarin

There are some documented cases of unwanted pregnancy that have occurred in women who added St. John’s wort to their birth control pills.

Vitamin C


Only a concern with very high doses of vitamin C; at least one study showed that taking up to 10 grams of vitamin C per day had no effect on warfarin’s anticoagulant effects.

Sources: Bob Rountree, MD; A–Z Guide to Drug-Herb-Vitamin Interactions by Alan R. Gaby, MD, et al. (Prima Publishing, 1999).