a pill with silverware Most of us have heard that certain medicines, when taken together, can be potentially harmful. But what about food and medicine? What possible harm could come from eating a cheese sandwich after taking an antibiotic? You may be surprised to learn that certain foods can dramatically affect your medicines.

Knowing how and when to take your medicines can eliminate or reduce interactions between food and drugs. Your pharmacist and your doctor can provide you with the most up-to-date information about these interactions.

Take this quick quiz to check your knowledge of food and drug interactions.

True or False?

Answer true or false for each of the following eight questions.

  1. Medicine should always be taken with meals.
  2. Only prescription drugs interact with food.
  3. It is safe to take my medicines with a glass of milk.
  4. I take high blood pressure medicine. Therefore, I should use a potassium-containing salt substitute.
  5. Mineral oil is a harmless, gentle laxative.
  6. Any pharmacy can fill my prescriptions.
  7. Grapefruit juice is a harmless, healthy source of vitamin C.

How Did You Do?–The Answers

1. medicine should always be taken with meals—False
The size and composition of a meal determines how quickly your medicine will be absorbed. Some medicines, such as aspirin and nonsteroidal anti-inflammatory drugs (eg, ibuprofen) irritate the stomach lining if you take them on an empty stomach. Other medicines, however, should be taken on an empty stomach. Food may slow their digestion or absorption. This is particularly true of some antibiotics.

2. Only prescription drugs interact with food—False
Over-the-counter medicines that you buy without a prescription, such as aspirin and low doses of ibuprofen, often interact with food. Watch out for alcohol, too. It blocks the effects of some drugs and intensifies the effects of others to dangerous levels. For example, alcohol combined with nitroglycerin can dangerously lower blood pressure. If you are taking antibiotic metronidazole (Flagyl), you may need to avoid alcohol completely.

3. It's safe to take my medicines with a glass of milk—False
Some drugs are negatively affected by dairy products. For example, the calcium in milk binds up tetracycline, a commonly prescribed antibiotic, so less tetracycline is absorbed. To prevent this, tetracycline should be taken at least two hours before or after eating dairy products or taking calcium supplements.

4. I take high blood pressure medicine. Therefore, I should use a potassium-containing salt substitute—False
This can sometimes be a dangerous misconception. Some blood pressure medicines (eg, furosemide) cause you to lose potassium, so your doctor may prescribe a potassium supplement. However, other classes of blood pressure medicines actually prevent potassium loss. If you take potassium-sparing diuretics or ACE inhibitors, avoid liberal use of salt substitutes that contain potassium. Excessive use of these products causes an accumulation of potassium, which can lead to severe complications that can threaten your health or life.

5. Mineral oil is a harmless, gentle laxative—False
Although gentle, mineral oil is a fat-soluble liquid. Mineral oil goes through the body undigested, robbing the body of fat-soluble vitamins like A, D, and E.

6. Any pharmacy can fill my prescriptions—True
Any pharmacy can fill your prescriptions. However, it makes more sense to fill all your prescriptions at the same pharmacy. This enables the pharmacy's computer to keep track of all your medicines so that the pharmacist can note any potential cross-reactions between existing medicines and new ones. By visiting the same pharmacy all the time, you also create a relationship with the pharmacist. This makes you more likely to discuss any concerns you have.

7. Grapefruit juice is a harmless, healthy source of vitamin CFalse
Grapefruit juice is healthy on its own, but it can interact with numerous medicines, potentially reducing their effects or increasing the risk of toxicity.

Other Food and Drug Issues

In addition to food and drug interactions, certain medicines also affect taste, sensation, and appetite. For example, penicillin can make foods "taste funny." Antihistamines and certain antidepressants can cause dry mouth, making it hard to chew and swallow. Certain pain medicines and iron supplements are a frequent cause of constipation.

How do you know when and how medicines should be taken? Read the directions printed on the container and ask your doctor or pharmacist. Food and drug interactions are almost always avoidable or manageable.

To keep food-drug interactions at a minimum, follow these tips:

  • If you experience any unpleasant new symptoms after taking a new medicine—or one you have been taking for a while—tell your doctor.
  • When a new medicine is prescribed for you, tell your doctor and pharmacist about any other medicines you routinely take, including herbal remedies, vitamin supplements, and nonprescription drugs.
  • Let your doctor know if you follow a special or restricted diet. Kosher diets have an unusually high sodium content, while traditional Asian diets may be high in both sodium and calcium. These factors may affect the types or even the brands of drugs prescribed for you.
  • Always take your medicine at the time and in the manner prescribed. Taking too much or stopping too soon can be dangerous.
  • With each medical check-up, review your medicines, lifestyle, and dietary habits. If you have recently lost weight or become a vegetarian, your medicine doses may need to be changed.