Drug interactions explained

In modern medicine, it is quite common for most of us to take some form of prescribed drugs to treat certain illnesses or medical conditions. The consumption of such medicine may be short-term or long-term, depending on the treatment duration for your condition. Did you know that certain drugs have the potential to interact with other drugs, causing side effects or reduced effectiveness of the drug?

Professor Gan Siew Hua, Professor in Pharmacology at the School of Pharmacy, Monash University Malaysia explained, “Quite often, when taking drugs, you may also be consuming other drugs, vitamins, minerals, and other common foods and beverages. It is important to note that some drugs when taken together with food, vitamin, or supplements, may result in drug interactions which could sometimes be harmful to the body.”

(Note: In this article, ‘drugs’ refer to medicines intended for use in the diagnosis, cure, mitigation, treatment or prevention of disease, and not the illegal substance that can cause addiction.)

What are drug interactions?

“Drug interaction happens when two or more drugs which are taken at the same time react with one another. When taken together, drugs may interact – either good or bad - with each other. Good interactions happen when a drug is able to reduce the side effects of other drugs or increase the therapeutic effect of other drugs, when both are taken together. On the other hand, bad interactions occur when the therapeutic effect of the drug is reduced, or creates a dangerous effect,” explained Prof Gan.

She added that besides interaction with other drugs, drug interaction may also occur with general drugs or those bought over-the-counter, herbal medicine and even certain food and drinks.

Interaction with general drugs, vitamins and supplements

“Something as common as a painkiller from the non-steroidal anti-inflammatory drug (NSAIDs) group such as aspirin, diclofenac and mefenamic acid may react with vitamin C (ascorbic acid). Since NSAID medication is also acidic, taking it together with vitamin C may increase the side effects of both substances,” said Prof Gan.

“It is also advisable to be alert if you are taking minerals such as zinc and iron. Some cholesterol-lowering medication such as cholestyramine and colestipol can reduce iron absorption. So, if you are taking iron supplements to treat anaemia (lack of red blood cells in the body), then the iron may not be absorbed into the body,” she elaborated.

She further explained that another similar interaction could happen between iron and certain medications used to treat ulcers or other stomach problems. The stomach acidity is affected and iron absorption is reduced if taken at the same time. Examples of these medications are cimetidine (TagametⓇ), ranitidine (ZantacⓇ), famotidine (PepcidⓇ) and nizatidine (AxidⓇ).

Some categories of antibiotics may also interact with iron, where the iron attaches to the antibiotics in the stomach and reduces the amount of antibiotics that the body can absorb. To avoid this, Prof Gan advised to space out the interval between the consumption of the iron and antibiotics.

Reduced effectiveness of antibiotics

Antibiotics are commonly prescribed for bacterial infections. Yet, one should be cautious not to mix selected antibiotics with other drugs, for fear of reducing the effect of the antibiotics.

“The absorption of antibiotics from the quinolone group (example ciprofloxacin and pefloxacin) can be reduced by gastric drugs such as sucralfate and aluminium or magnesium-containing antacids. A similar effect also happens when taken together with drugs containing calcium, iron and zinc,” said Prof Gan.

Drugs for high blood pressure and blood thinning

Drugs for high blood pressure, heart conditions and stroke are often consumed for a long period of time. Precaution should be taken when these drugs are taken with other drugs.

“Patients who are consuming blood thinning drugs such as warfarin (CoumarinⓇ) should take note never to consume together with paracetamol or more commonly known as PanadolⓇ in this country. Paracetamol, especially if taken more than the recommended dose, can increase the risk of excessive bleeding if taken with warfarin. This may result in nose or gum bleeding, increased bruising or dark-coloured stools due to the presence of blood in the stools. However, the effects are more severe if warfarin is taken with a non-steroidal anti-inflammatory drug like ibuprofen”.

“Another example of drug interaction is the interaction between potassium sparing diuretics and medicine used to treat high blood pressure from the group ACE (angiotensin-converting enzyme) inhibitors (example captopril, enalapril and perindopril). When these are taken together, they cause increased levels of potassium in the body, which, when accumulated in high amounts, can cause irregular heart rhythm,” she continued.

Sleeping pill interactions

Prof. Gan also explained that if you are consuming sedatives or sleeping pills from the benzodiazepines group (example alprazolam, diazepam and midazolam), be cautious when you are mixing the sleeping pills with other types of drugs. The drowsiness effect may be increased when it interacts with other drugs from many categories.

These include: antifungals (example ketoconazole and miconazole), gastric drugs (cimetidine), antibiotics (erythromycin) and allergy drugs from the antihistamine group (chlorpheniramine - PiritonⓇ).

Interactions with epilepsy drugs

A type of epilepsy drug in particular -- phenytoin -- reacts negatively with certain drugs.

The negative side effects of phenytoin may increase when consumed together with cimetidine, a type of gastric drug, and different types of antibiotics such as erythromycin (ErycinⓇ), clarithromycin (KlacidⓇ) and antibiotics from the sulphonamides group, e.g. co-trimoxazol (BactrimⓇ). The side effects include excessive sleepiness, headache, nausea, vomiting, difficulty in sleeping and allergies.

Phenytoin should also not be taken together with other epilepsy drugs such as phenobarbital, carbazepine and valproic acid as the level of phenytoin may be reduced and affected, causing seizures.

Pills affecting couple relationships

Prof. Gan next touched on the topic of drug interactions with contraceptive pills and pills for impotence or erectile dysfunction.

“Women who are taking contraceptive pills with the purpose of preventing pregnancy should take note that certain drugs can reduce the effectiveness of contraceptives. So, do take note not to take these together, or you may suddenly find out that you are pregnant! Seek advice from your doctor or pharmacist if you are adding new drugs to your daily regime,” she cautioned.

Drugs which may react with contraceptive pills include drugs to treat tuberculosis such as rifampin, and drugs for epilepsy, including carbamazepine, phenobarbital and phenytoin, as described earlier. The reduction of the contraceptive effect is more likely to happen with the combination type of pills, or COC, such as YasminⓇ, MercilonⓇ and MarvelonⓇ. Likewise, the oral contraceptives reduce the level of phenytoin in the blood, therefore the patient may suffer from seizures.

Further examples on drug interactions

In the book by Professor Gan, ‘Tips on Using Common Medicines Safely’, you will find a table listing bad drug interactions that one should be cautious of when consuming drugs with other drugs. Interactions although possible, may not necessarily occur in all individuals since drug interaction varies from one person to another.

When prescribed any medicine, it is highly advisable to inform your doctor or pharmacist of the full range of drugs and supplements that you are currently taking to avoid any negative side effects.