Avoiding drug interactions: Here’s help

Consider a “brown bag review” for patients who don’t know what dosage they’re taking or have difficulty identifying the drugs other physicians have prescribed. Ask them to put all their medications in a brown bag and bring them in on their next visit.^14,15

Steps to take to reduce risk

Software systems. A number of free and low-cost software systems identify potential DDIs (See “Check for drug interactions: Software programs to consider”). While such electronic programs can indeed lower the risk,^16-18 they cannot be counted on to detect or avert every possible adverse interaction.

The downside. One problem is that some software programs fail to distinguish between clinically significant and nonsignificant interactions, causing some prescribers to override system alerts—and possibly miss an important warning.¹⁹ Another problem: While most systems do an excellent job of checking to see whether 2 drugs can be safely taken together, few are capable of checking for all potential interactions among multiple medications. What’s more, many drugs have not been evaluated for their potential to interact with other agents, so the absence of reported interactions is no guarantee of a lack of DDIs.

Other strategies to consider:

Minimize the number of prescriptions. While it may not be possible to avoid prescribing a new agent for a patient who is already taking multiple medications, limiting the number of new drugs to those that are absolutely essential will help to minimize DDIs. Whenever possible, select a compound with the desired effects. Prescribe a single agent with antihypertensive as well as uricosuric effects for a patient with elevated blood pressure and uric acid levels rather than 2 different drugs (eg, losartan instead of an anti-hypertensive agent plus allopurinol).

Alter the dosing regimen. Several active molecules may cause DDIs by interfering with intestinal absorption or GI transit time if they’re taken closely together. For example, a quinolone should not be administered at the same time as a cation because of possible chelation in the GI tract. If a patient needs both, however, you may be able to avert a DDI by advising the patient to take them at least 2 hours apart.²⁰ Another possibility is to temporarily discontinue a maintenance medication if it has the potential to interact adversely with a drug that is needed for only a short duration.

Choose a different drug (or drug class). Some drug classes should never be mixed—nitrates and phosphodiesterase type-5 inhibitors, taken together, greatly increase the risk of vasodilation and may result in severe hypotension, for example. There are also drug classes with a low potential for DDIs, including cholinesterase inhibitors and anti-hypertensives (angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, beta-blockers, and thiazides).²¹

Frequently, though, medications within the same drug class do not share the same potential for DDIs. In such cases, an adverse outcome can often be averted by being aware of combinations likely to result in clinically significant DDIs (TABLE 2) and, whenever possible, prescribing another agent. If a patient taking carbamazepine needs a macro-lide antibiotic, for instance, azithromycin is a better choice than erythromycin. That’s because erythromycin inhibits the hepatic metabolism of the anticonvulsant, increasing the serum level of carbamazepine, while azithromycin does not interfere with carbamazepine metabolism.²²

TABLE 2
Clinically significant drug-drug interactions^21,22