Excessive intake of vitamin K may antagonize the effects of warfarin. Another class of drugs that can interfere with quite a few vitamins are oral birth control pills. The progesterone component may reduce plasma levels of folic acid, vitamins B12, B6 and B2. Oral contraceptives can also increase the bioavailability and half-life of caffeine. This last fact may be of clinical significance in our coffee-dependant West Coast.
Herbs are just as likely to be at fault.
Garlic can enhance the effects of anti-coagulant medications. There is the theoretical possibility that ginkgo can enhance the actions of anti-coagulants as well. Panax ginseng can have an additive effect when taken with MAO Inhibitors. Long-term consumption of kava, anywhere from a few months to a year, can result in dry scaly skin. One theory behind this adverse effect is that kava may interfere with the absorption and assimilation of B vitamins.