The medication that is taken by mouth travels through the digestive system in much the same way as food and herbs taken orally do. So, when a drug is mixed with food or another herb, each can alter the way the body metabolizes the other. Some drugs interfere with the body's ability to absorb nutrients. Similarly, some herbs and foods can lessen or increase the impact of a drug. Alcohol is a drug that interacts with almost every medication, especially antidepressants and other drugs that affect the brain and nervous system.
Some dietary components increase the risk of side effects.
Theophylline, a medication administered to treat asthma, contains xanthines, which are also found in tea, coffee, chocolate, and other sources of caffeine. Consuming large amounts of these substances while taking theophylline increases the risk of drug toxicity. Certain vitamins and minerals impact on medications too. Large amounts of broccoli, spinach, and other green leafy vegetables high in vitamin K, which promotes the formation of blood clots, can counteract the effects of heparin, warfarin, and other drugs given to prevent clotting. Dietary fiber also affects drug absorption.