October 29, 2010

One of the most common types of questions I encounter are those dealing with drug interactions. I thought it may be helpful to write a little bit about how some drug interactions occur. I’m not going to provide many specific examples, so to keep this in broad scope please understand that these issues can arise between drugs, herbal products, vitamins, foods, alcohol, and smoking. There are several distinct ways that the effects of a medication can be altered, and I’ll discuss some of them today.

Probably the most common issue that arises is a change in the way a drug is metabolized (altered in a sense to either an inactive or active form). There are many enzymes in the body that break down medications. Numerous medications can increase or decrease the activity of these enzymes. If medication A increases the activity of an enzyme that breaks down medication B into an inactive form (or a form that is more readily eliminated from the body), the end result will be an increase in the active form of medication B. This will result in increased effectiveness and/or increased toxicities of med B. Most often times it is the latter, so this is usually an undesirable effect. Now lets imagine that med A decreases the activity of an enzyme that activates med B. The end result here will be a decrease in the effectiveness of med B. A great example of this is the developing concern that drugs like Prilosec may decrease the effectiveness of the blood thinner Plavix, because they decrease the activity of the enzyme that changes Plavix into its active form. There are all sorts of enzymes and transporters in the body that can be effected, so these metabolism interactions are a very common problem.

Another common type of drug interaction occurs when the absorption of a medication is altered. This occurs commonly with foods and vitamins. Taking some medications with food can increase their absorption (and thus their effects). Some medications have decreased absorption with food. An example of this would be some pain medications, where taking them with food might decrease side effects of nausea (and the effectiveness of relieving pain). Taking calcium, magnesium, or iron with some antibiotics can decrease their absorption and effectiveness. Some drugs can do this too, especially cholestyramine and sucralfate.

Aside from changes in a way the medication is handled inside the body, drug interactions can occur due to additive or counteractive effects of medications. If one medication causes sedation, and you add another that does so as well, those effects will be magnified. These interactions can be particularly troublesome with medications that can affect breathing, heart function and rhythm, and those that cause organ toxicities. On the other hand, medications might work against each other. For example, some beta-blockers can decrease the effect of albuterol inhalers (since the albuterol is trying to hit the beta-receptors that are being blocked).

As you can imagine, there are thousands of drug interactions that occur due to the above mechanisms (amongst some others as well). Sometimes these effects are purposeful and beneficial, but many times they can be dangerous and require strategies to prevent complications. You should always check for an interaction if you are taking more than one substance (drug, herb, vitamin, etc.). Ask your pharmacist to explain the effects as well, and for a recommendation on how the problem should be addressed. You can always submit a question on my blog as well.