5 anxiety disorders

PHARMACOLOGY 31the bacteria more effectively than either could alone. Likewise, psychiatric medicationsthat work in different but complementary ways are sometimes combined toachieve a better treatment response.So why all the hubbub about drug interactions? This is because drug interactionscan also produce harmful effects. And if the prescriber, or the patient, doesn’t seethem coming, the results can be disastrous. For example, a few years ago, the nonsedatingantihistamine astemizole (Seldane) was introduced, essentially revolutionizingthe treatment of seasonal allergies. In FDA testing, it proved a safe andeffective medication. But later, in routine clinical use, it was not uncommon forpatients to take the antibiotic erythromycin together with Seldane. The problem isthat erythromycin interferes with the liver’s ability to metabolize Seldane. As aresult, Seldane accumulates when taken with erythromycin. The danger arisesbecause extremely high levels of Seldane can lead to heartbeat irregularities, thatis, cardiotoxicity. Consequently, two “safe” medications, taken together, combine toproduce a lethal mixture.We don’t, however, have to proceed blindly when two or more medications areprescribed together. If we understand the pharmacokinetics and pharmacodynamicsof each medication, then we can predict with reasonable certainty the likely interactions.Let’s take a look at how such interactions can occur.Pharmacodynamic Interactions. Sometimes medications interact pharmacodynamically.If two medications produce similar side effects, then those effects canbe additive. This can be advantageous. For examples, coadministering two antidepressantsthat relieve depression in different ways can be more effective thaneither medication alone. However, added effects can be problematic. If two medicationsthat each produce drowsiness are coadministered, then the combination mayproduce intolerable daytime sedation.On the other hand, the effects of two medications can counteract one another.The result is usually that both medications are rendered less effective. A commonexample is the patient with Parkinson’s disease. On occasion, the L-DOPA that isthe mainstay of treatment causes hallucinations. The treatment for hallucinations isan antipsychotic, which blocks the activity of dopamine. The problem is that usinga typical antipsychotic to treat L-DOPA-induced hallucinations will interfere withthe therapeutic effect of the L-DOPA, thereby worsening the symptoms of theParkinson’s disease. Fortunately, the advent of the newer atypical antipsychoticshas provided a remedy to this particular Catch-22 drug interaction dilemma.Pharmacokinetic Interactions. Remember that pharmacokinetics is the studyof what the body does to the medication. This includes how the body transports,inactivates (i.e., metabolizes), and eliminates the medication. Sometimes, onemedication can change the way the body handles another medication. This is knownas a pharmacokinetic interaction. There are two types of pharmacokinetic interactions.One type occurs in the medication transportation system, and the otheroccurs in the medication elimination system.