Medical Conditions Related to Brain Injury
Drug Interactions
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“Drug interactions” occur when a person takes 2 different medications or substances which react together in the body to produce unintended results. They may result from any combinations of prescription drugs, over-the-counter medications, dietary supplements, herbal products, illegal drugs of abuse, smoking cigarettes, and even some foods. As the number of medications a person takes increases, so does the potential for interactions.
There are many different mechanisms by which drug interactions occur. Some interactions cause a decrease in the amount of a drug in the body to the point that the medication becomes ineffective.
An example of this type of interaction is with the anticonvulsant drug carbamazepine (Tegretol). Carbamazepine works on various liver enzymes to speed up the elimination of many other medications, including oral contraceptives and some other anticonvulsants, so that an inadequate amount of medication is present in the body to be effective. Possible consequences when these drugs are given with carbamazepine include unplanned pregnancy and loss of seizure control, respectively. These medication combinations can be used together but dosage increases are usually necessary to achieve optimal results with the medication that is being affected by carbamazepine. Other drugs which can cause this type of interaction and decrease levels of certain other medications include phenytoin (Dilantin), phenobarbital, some HIV antiviral drugs, rifampin, prednisone, some antacids, and the herbal product St. John’s Wort. Smoking can also significantly decrease levels of a few medications, including olanzapine (Zyprexa), so that higher-than-normal doses are needed to achieve effective results.
Another type of interaction is when one drug or substance causes an increase in the amount of another medication in the body, sometimes to dangerously high levels (called “toxicity”). The classes of medications most likely to cause these types of interactions include antidepressants, antifungal agents, antibiotics, HIV antiviral drugs, some anticonvulsants, and some calcium channel blockers. Grapefruit juice can also interact in this way, which is why there are warnings against its consumption on some prescription medication labels.
A third type of interaction is when drugs have added pharmacological effects. For example, when a benzodiazepine such as alprazolam (Xanax) or clonazepam (Klonopin) is combined with drinking alcohol, their depressant effects are more than additive. Each can cause sedation when taken alone, but their concurrent use can suppress breathing and cause death. This is an example of a drug combination that should always be avoided.
Much new information has been learned about drug interactions during the past decade, and most significant interactions with prescription drugs are preventable today. Each person taking medications should have their complete regimen (including all nonprescription products) reviewed for interactions by both a physician and a pharmacist. Those who see more than one physician should provide a complete medication list to each doctor. Always consult with your physician or pharmacist before taking any new over-the-counter medication or dietary supplement. Lastly, it is important that all prescriptions are filled in one pharmacy so that the pharmacist has a complete record and can monitor for drug interactions.
Sara Grimsley Augustin, Pharm.D. and Board-Certified Psychiatric Pharmacist (BCPP), is on faculty at Mercer University Southern School of Pharmacy in Atlanta, GA, where she teaches in the areas of psychiatry, neurology, and substance abuse. Dr. Augustin has provided consultant pharmacy services to Restore Neurobehavioral Program since 1992 and has worked with this program to optimize outcomes in neurobehavioral pharmacotherapy.