Medical Drug Interactions

When two or more drugs are taken at the same time a drug interaction can occur. These interactions can also occur between pharmaceutical drugs and herbal medications or even between drugs and foods. The greater the number of drugs and/or herbs taken the greater the possibility of interactions.

What are drug interactions?

An interaction is a generally unwanted result of combining two or more chemicals within the body. Sometimes it involves the result of direct chemical interactions (such as when two chemicals combine to create a third and unplanned chemical) and sometimes it relates to the impact on the body caused by treatment with more than one drug at a time, such as two drugs affecting the same system, even though the drugs don’t themselves combine to create yet another drug. Drug interactions are very common and while many have only minor effects they can be quite dangerous, even lethal.

Drug interactions are usually characterised by one of two general effects. One involves the drugs enhancing their effects (that is potentiating or working together to have a stronger action). For example, drugs that are used to thin the blood and reduce the possibility that the blood will clot (anticoagulants) are commonly affected by this problem. Combined treatments will work together and the blood could become so thin that bleeding could occur and it would be difficult for the blood to form a clot to stop the bleeding. One example would be taking aspirin while being treated with heparin since both are known anticoagulants. There are also herbs that have an anticoagulant effect and these can interact with medical anticoagulants. These herbs include cayenne and ginger which are specific herbal anticoagulants. There are other herbs and vitamins that have an anticoagulant effect – these include ginkgo biloba, garlic and vitamin E. The other general effect of course is just the reverse. This involves the drugs working against each other causing them to be less effective.

How do the drug interactions occur?

There are many ways that the interactions can take place. The main ways are covered below.

Physical or chemical incompatibility – This usually occurs when drugs are mixed in the liquid form. If there is a physical incompatibility then there is often a precipitate – solid parts start forming in the mixture. An example is diazepam (Valium) – a common sedative. In the liquid form it will not mix well with water. This is a physical incompatibility. With a chemical incompatibility a chemical reaction can occur which alters the drug. For example, the liquid form of penicillin (an antibiotic) will be inactivated if it is mixed with aminoglycosides (another group of antibiotics) such as streptomycin.

However, this type of chemical incompatibility can be beneficial. For example, the drug action of the blood thinner heparin can be reversed by the drug protamine, a drug which binds to and inactivates the heparin. Protamine is therefore used as a specific antidote for a heparin overdose.

Interference with the absorption of nutrients in the digestive tract – The rate at which the stomach and intestines work can be affected by various drugs.

For example, when anti-nausea drugs such as metoclopramide (Maxolon, Pramin) are taken, they will speed up the rate at which the stomach empties. This is important to know if, for example, digitalis (a heart drug) is given as the time that it has to be absorbed in the stomach is reduced and the effective dose will therefore be lower. If the stomach emptying time is lowered, for example, with propantheline (an antispasmodic drug) the time spent in the stomach, and hence the absorption of digitalis, will be increased.

Changing of drug metabolism – Some drugs are able to stimulate the breaking down of other drugs in the liver.

For example, barbiturates (anti-anxiety sedatives) are known to do this. When barbiturates are given with oral anticoagulants (blood thinners) then the dose of the blood thinners needs to be increased because of the rate at which they will be broken down in the liver. There are many examples of where one drug will affect the rate at which another drug is broken down.

Drugs which affect the same body system – This is where two drugs have an effect, which may be intended or otherwise, on the same body system.

For example, drugs that dull the responses of the brain. These include: alcohol, antihistamines, sedatives and pain killers. The effects of one will tend to make the other stronger. This can also work by a drug reducing the effect of another drug. For example, drug doses of vitamin K works against the effects of oral blood thinners.

Drug and food interactions – The importance of these interactions should not be overlooked.

For example, Monoamine oxidase inhibitors (MAOIs) (such as Phenelzine, Iproniazid, Tranylcypromine) are antidepressant drugs that interact with foods rich in tyramine. The result can be a rapid and prolonged rise in the blood pressure. Foods that have to be avoided include: avocados, cheese, spirits and wine, pickled herring, beef extracts and broad beans. Some over-the-counter drugs such as cold remedies and nasal decongestants can have a similar interaction with the MAOIs.

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