In order to work properly, drugs must be present in the correct amounts at the receptor sites of the cell membranes. A number of factors can affect the way in which a drug works and, therefore, its effect on the body. Food is one of the most important of these factors. It can affect the absorption of a drug, its movement through the blood, its metabolism and its excretion.
Food and drug absorption
When drugs are taken orally, their absorption is influenced in different ways by the presence of food in the digestive tract. For example, food causes increased secretion of hydrochloric acid (HCl) in the stomach which can result in the destruction of some penicillin antibiotics that are taken by mouth. For this reason, penicillin should always be taken 1 hour before or 3 hours after meals.
Calcium in milk and dairy products tends to bind with tetracycline antibiotics to form non-absorbable compounds, with the result that the drug is lost in the stool. This means that these foods should not be eaten at the same time or within a few hours of taking the medication. However, the instruction 'Not to be taken with milk products' which sometimes appears on medicine labels can be misleading as it does not mean that you cannot consume any milk products while you are on the medication.
Iron salts (such as ferrous sulphate) can also bind with tetracycline antibiotics and result in the loss of the antibiotic. Therefore, anyone using an iron supplement as well as a tetracycline should take them at least two hours apart.
On the other hand, the absorption of some drugs can be increased when they are taken with foods. For example, foods with a high fat content increase the absorption of Griseofulvin, an antibiotic and antifungal drug. Iron supplements are also absorbed better when they are taken with vitamin C or foods rich in vitamin C, such as orange and guava juice. Interestingly, although some drugs (such as aspirin) are absorbed better on an empty stomach, they should nevertheless be taken with food - or at least milk - because of their irritating effect on the stomach lining.
Food and drug metabolism
Certain reactions inside the body can chemically change a drug (`metabolize' it) so that it is quite a different compound by the time it leaves the bloodstream. For example, some drugs combine with body proteins as they move through the bloodstream, which has the effect of preventing these drugs from entering the cells where they were intended to act. Others are broken down into smaller chemical components by the liver, the kidneys and the digestive tract. The metabolism of specific drugs can be influenced by the following in food: protein content, alcohol content, charcoal broiling, certain types of vegetables, caffeine intake, dietary fibre and the times food is eaten in relation to drug intake.
Food and the excretion of drugs
When metabolized by the body, some food alters the pH of the urine making it either more or less acidic - which in turn affects the excretion of drugs and therefore, the duration of their effects. For example, if excretion is decreased, more of the drug will be reabsorbed into the blood and its effects will be prolonged.
As an illustration, vitamin C makes the urine more acidic, thereby reducing the excretion of acidic drugs like phenobarbital and aspirin. When taking an antibiotic for a urinary tract infection, it is beneficial to acidify the urine, because this will result in decreased excretion of the antibiotic.
What drugs should you take when?
In order to minimize the adverse effects of foods on drug absorption, metabolism and excretion, the timing of drug administration in relation to food intake is very important. This is why it is essential to adhere to any such instructions which may accompany a prescribed drug. For example:
- If rapid absorption of the drug is desired, the doctor/pharmacist may specify on the prescription that the drug should be taken before meals.
- Drugs that irritate the stomach (such as indomethacin, phenobarbitone) should be taken when the stomach is 'cushioned' with food. In this case, the prescription should specify that the drug is to be taken after (or in the middle of) a meal.
- Fat-soluble drugs should not be taken with a fatty meal, since the medication will be absorbed with the fat content in the food; because fat is digested slowly, the drug may not be absorbed into the bloodstream as rapidly as it should be.
- Acetylsalicylic acid, Bisacodyl, Erythromycin and tetracyclines should not be taken with milk (see above).
- Ampicillin, Cloxacillin and Erythromycin-based drugs, as well as stearate and Penicillin G should not be taken with fruit juices or carbonated beverages.
