Drug interactions with food can make drugs more or less effective, and can make drug side effects worse, but why does this happen?
Delaying Stomach Emptying
Many foods can slow down the rate that the stomach empties part-digested food into the small intestine, where drugs and nutrients from food are absorbed.
Some drugs need to be absorbed quickly to have an effect, and the slower emptying slows the rate at which they are absorbed, so makes them less effective.
Slower emptying means that drugs taken by mouth stay exposed to the acid in the stomach for longer. If a drug is broken down and destroyed by stomach acids, the drug staying longer in the stomach means that there will be more of the drug destroyed and less of the drug remaining to have an effect. The effects of the acid in the stomach (or even the acid in the drinks taken with the drug) can make some drugs more soluble in water, which helps them be absorbed more quickly – if drugs stay in the stomach longer, more of the drug becomes soluble, so the drug can become more effective, or show more side effects.
Some foods, especially fibre, can bind to drugs, so limiting the amount of drug absorbed and reducing its effects.
Some foods can reduce the absorption of drugs, so making them less effective.
Reducing the Breakdown of Drugs
Enzymes in the liver break down many drugs, which helps the body to get rid of the drugs once they have done their job. Some foods, such as grapefruit juice, stop these enzymes working. This means that drugs are not broken down as efficiently and levels in the body increase, which could make the drugs too effective, or make side effects worse.
Changing the Metabolism of Food
Some drugs block the action of enzymes that break down food. For example, an enzyme called monoamine oxidase breaks down tyramine, which is found in foods like cheese, yeast extract, soy sauce, wine and beer. Monoamine oxidase inhibitors are used to treat depression. They block this enzyme, so if people taking monoamine oxidase inhibitors eat foods high in tyramine, they can end up with high levels of tyramine, causing high blood pressure.
Reducing Stomach Irritation
Some drugs have to be taken with food to reduce irritation on the lining of the stomach, causing stomach upsets (see ‘Timing of Drugs – Before or After Food’).
The Benefits of Food-Drug Interactions
As some food interactions increase the effectiveness of some drugs, it is possible that this could be used to reduce the dose required, which could reduce the costs of the treatment, and any side effects. Patients should only do this at the advice of a doctor.
What to Do?
When taking a drug for the first time (whether it is prescribed or just bought over the counter), always check on the information sheet or package to see if it interacts with any particular foods. If in doubt, always ask the doctor or pharmacist.