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Interactions can be deadly

Mixing household cleaners can be deadly. Putting ammonia and bleach together generates potentially lethal gas.

Mixing medications can be equally dangerous. Yet physicians still prescribe combinations such as Viagra and nitrate heart medicines or the antibiotic cotrimoxazole (Bactrim or Septra) with a blood pressure drug like Diovan (valsartan). Such mismatches can prove fatal.

In one case, a man who had received a heart transplant was taking drugs to prevent rejection of his new organ. They included azathioprine, cyclosporine and prednisone. His primary-care physician detected high uric-acid levels and prescribed allopurinol to lower them. This landed the 52-year-old in the hospital with a life-threatening blood disorder (Pharmacy Times online, July 1, 2006).

This potentially deadly combination should never have happened. The danger of this interaction has been known for more than 40 years. But doctors don't always recognize which drugs should never be used together.

When prescribers were tested about their knowledge of drug interactions, they performed miserably (Drug Safety, June 2008). They were given 14 drug pairs to classify as forbidden, risky or not a problem. They got the right answer on fewer than half the pairs. Any high school student who scored below 50 percent on a test would fail.

It isn't hard to understand why doctors can't remember dangerous drug interactions like the anti-anxiety agent alprazolam (Xanax), which should never be mixed with the antifungal medicine itraconazole (Sporanox). With thousands of medicines available, the number of bad combinations could reach into the thousands. This is beyond the capability of human memory.

That's why there are computers and smartphones. Many physicians and all pharmacists now rely on electronic databases for writing and filling prescriptions. These tools provide warnings about drug incompatibility. But a recent study found that even when physicians had access to a wireless handheld device with interaction information, they were just as likely to prescribe a bad combination (Journal of Managed Care Pharmacy, January-February 2012).

Whether the drug in question is lisinopril for hypertension, simvastatin for high cholesterol, sertraline for depression or warfarin to prevent blood clots, patients and their families need to be alert for possible dangerous combinations. Don't assume the prescriber is aware of all the drugs a patient is taking or the hazards of mixing medicines. Our book "Top Screwups Doctors Make and How to Avoid Them" (online at www. has tips for preventing deadly drug interactions.

Pharmacists may catch hazardous combinations before they are dispensed, but like prescribers, they also may override computerized alerts. Even when they detect serious problems, it can be challenging for them to communicate directly with the prescriber and arrange for a less dangerous medication.

Anyone who takes more than one pill at a time should always trust but verify. Ronald Reagan's favorite slogan applies to medicine as much as to international politics.