Understanding Drug-Food Interactions

When a food alters a drug’s activity or vice versa, the affected compound is changed in aspects of how quickly or how much of it gets absorbed into the body, how much of it distributes to it’s site of action, and how quickly it gets broken down or passed out from the body. Such drug-food interactions may result in the individual experiencing side effects, toxicity or not getting full benefit from the drug or food.

In altering the absorption of drugs, foods may bind with the drug, change the acidity of the stomach, moderate the contractions of stomach muscles or change the speed at which stomach contents move through the body. Dietary fibre in the form of pectin and other soluble fibres can slow down the absorption of Tylenol® (acetaminophen). Almost half of a single dose of the antibiotic Zithromax® (azithromycin) does not get absorbed when it is taken with food. When another antibiotic ciprofloxacin is taken with iron, a complex is formed and about half of a single dose also becomes unavailable for absorption. One of the most significantly food-affected drugs is Fosamax® (alendronate) where up to 99.9% of an oral tablet can become unavailable for absorption if taken with food, coffee or orange juice. Sometimes increased benefit is derived from taking certain medicines with food. Taking the statin Mevacor® (lovastatin) with food enhances it’s absorption while taking non-steroidal anti-inflammatory drugs like aspirin with food can help to reduce stomach irritation.

Different drinks can alter the breakdown of a drug in the body. Components of grapefruit juice can inhibit enzymes in the liver from breaking down a variety of drugs. Taking grapefruit juice while on the heart drug felodipine results in much less of the drug broken down and this can cause lower blood pressure and faster heart rates than otherwise expected. The heart drug Inderal® (propranolol) can escape early breakdown in the liver when it is taken with food hence it’s effectiveness could be increased.

One important class of drugs known as monoamine oxidase inhibitors (MAOIs) can inhibit body enzymes from breaking down a compound known as tyramine. This compound can be found in aged cheese, pickled fish, yeast extracts, red wine, fava beans and fermented products, and can accumulate in the body if taken with MAOIs and the combination could result in critically high blood pressure.

Milk, vegetables and citrus fruits can reduce the acidity of urine while meats, fish, cheese and eggs often increase the acidity of urine. These different effects can affect the quantity and rate of expulsion of certain drugs. Other drugs may be more or less active in the urinary system depending on the urine acidity. The mood controlling drug known as lithium is affected by a high salt (sodium chloride) diet because it competes with sodium for absorption back into the body from the kidney. With a high salt diet, more lithium is passed out and lithium is consequently less active in the body. The converse occurs with a low salt diet.

Some foods can directly affect the action of drugs without affecting the drugs themselves. Taking a lot of green leafy vegetables or green tea can increase vitamin K levels in the body that counteract the effects of the anticoagulant drug warfarin. Taking a lot of bananas while on the potassium-sparing water pills Aldactone® (spironolactone) can result in excessive amounts of potassium in the body that can be toxic. Alcohol can increase the drowsiness and reduced concentration associated with sleeping pills, antidepressants and allergy medicines. Another potentiating effect can occur when the asthma drug theophylline is taken with coffee. The caffeine in coffee shares the same structure as theophylline and helps with expanding the airways but also can increase theophylline’s side effects of nervousness, tremor or insomnia.

While most drug-food interactions can be detrimental, some have been used under supervised treatment to reduce drug doses and associated costs. Oncologists recently found that taking the breast cancer drug Tykerb® (lapatinib) with food instead of without food as suggested by the drug development trials almost doubled the absorption of the drug and taking it with a high-fat meal tripled the absorption. Grapefruit juice may even further prevent the drug from being broken down in the body so that more of the dose remains available to fight cancer cells. While this is a novel and potentially useful means of taking food and drugs together for better treatment, expert supervision is important in ensuring benefit and minimizing undesirable effects.

Even while much of the concern over drug-food interactions center around how foods affect the activity of drugs, many drugs also expected to exert significant effects on foods, body nutrients and byproducts. The weight loss drug Xenical® (orlistat) is taken specifically with meals to interact with the fat in food so that it is not absorbed and passes out in the stools. Colestid® (colestipol) and Questran® (cholestyramine) are taken orally to bind bile acids in the intestine preventing such acids from being recirculated in the body and thereby reducing cholesterol. Another drug acting on food is Fosrenol® (lanthanum carbonate) that is used to bind and prevent the absorption of phosphates in the diet.

Some drugs can also affect nutritional well being resulting in the need for food or nutrient supplementation. Examples include taking certain antacids and water pills that can lead to nutrient deficiencies resulting in muscle weakness and other related symptoms. Folic acid supplementation may be required if on the anticonvulsant Dilantin® (phenytoin). Zinc deficiency coupled with the loss of taste can occur when on the antirheumatic penicillamine.

Drug-food interactions are becoming increasingly common and can be detrimental if not well understood or properly managed. Pharmacists are the experts to turn to for a better understanding and are in the best position to advise on optimum management.