Are your drugs compatible?

Some drugs may not do their job effectively if they’re taken with other drugs, supplements or certain foods. Here are some examples of combinations that may stop your medication from doing what you need it to do.

Drug interactions account for three to five per cent of preventable unwanted drug reactions in hospitals. Interaction occurs when one drug interferes with how another drug works.

So, what can you do? As a consumer, you need to know what drugs you should avoid or use cautiously when you are on a certain medication and to know what can happen when you combine two incompatible medications. Keep in mind that the cause of dangerous interactions is not just from taking prescription medications together. Combining over-the-counter medications, such as cough and cold products and herbal remedies such as St. John’s wort, even grapefruit juice with prescription medications can be dangerous. Here’s what you should know.


Consuming one glass of grapefruit juice is enough to affect certain medications for up to three days. Eating the fruit or drinking the juice can prevent certain drugs from being broken down in the body, increasing the amount of drug present in the blood. When the amount of drug in the blood is higher than what it should be, there is an increase in unwanted side effects and an increase in the effect the drug has on your body.

Take, for example, felodipine (Renedil or Plenedil) and nifedipine (Adalat XL), drugs used to treat high blood pressure. In the presence of grapefruit juice, your blood pressure is lowered even more and can cause lightheadedness, dizziness and fainting.

Lovastatin (Mevacor), atorvastatin (Lipitor) and simvastatin (Zocor) lower blood cholesterol levels but combined with grapefruit juice, headaches, stomach upset or serious muscle pain are signs of the drug’s toxicity and requires immediate medical attention. So before you go for a drink of grapefruit juice (fresh or frozen), the fruit itself, tangelos (a hybrid fruit containing grapefruit) or sour orange juice from Seville oranges, consult with your pharmacist to ensure what you are taking is compatible. Lemons, limes, tangerines and oranges are considered safe.


Gingko, garlic and ginseng – all herbal remedies – have blood-thinning properties. Anyone taking the prescription medication warfarin or the over-the-counter baby Aspirin to thin their blood should not take gingko, garlic or ginseng supplements without discussing it with a health-care professional. The herbal supplements may increase the risk of bleeding, so if you take them along with ASA or warfarin, the likelihood of bleeding increases.


If you use ibuprofen (Advil and Motrin) to relieve aches and pains, it can prevent ASA from doing its job. Both ASA and ibuprofen compete for the same binding site in your body. Think of the binding site as your front door. Whichever of the two drugs arrives at the door first is allowed in to do its job. The other is locked out. As ASA is taken daily to prevent a stroke, you do not want to prevent it from working. Take ASA at least two hours before you take ibuprofen in order to allow it to work. This applies to individuals who use ibuprofen occasionally, but if you take it on a regular basis, talk to your doctor as you may need to switch the ibuprofen to another pain reliever such as acetaminophen.