Rate of PDE-5 inhibitor-nitrate co-prescription higher than expected

Durham, NC-A study estimating the dispensation rate of interacting medications to an outpatient population found that while less than .05% of patients received co-prescriptions for medications that could potentially cause serious adverse effects, about half of these were for sildenafil citrate (Viagra) and one of two different nitrates.

Prescription of sildenafil and any nitrate medication, used to treat chronic and acute cardiac chest pain, is contraindicated because of the potential for a dangerous drop in blood pressure.

The study objective was to estimate the prevalence of absolutely contraindicated medication co-prescriptions among outpatients. Data were obtained from one of the largest pharmaceutical benefit management companies in the United States with beneficiaries in all 50 states, representing approximately 1,200 insurance companies.

The study defined a Level 1 interaction as co-prescription of medications that should never be prescribed together (according to the manufacturer's prescribing information), while overlapping prescriptions were those requiring a minimum 7 days of overlap, Dr. Scales said. Records were reviewed for a total of 4,825,345 patients, with a mean age of 47.5 years; 15% were 65 years or older. The study group was 41.9% male and 58.1% female.

All prescription claims were examined, and a subset comprising 800,000 patients had at least two claims. Of those, 376 patients (0.05%) had two prescriptions filled for drugs that should never be prescribed together (class 1 interactions).

Surprising proportion of co-Rx

In this group of 376 patients, 263 patients (65.9%) filed claims for sildenafil and a nitrate medication, with 199 prescriptions (30.2%) for sildenafil and nitroglycerin, and 127 (19.2%) for sildenafil and isosorbide nitrates. However, co-prescription with a nitrate occurred in only a small fraction of all sildenafil prescriptions. Because the study was conducted in 1999, only sildenafil was studied; it was the only phosphodiesterase type-5 inhibitor available for the treatment of erectile dysfunction at the time.

Nitrates (isosorbide mononitrate and nitroglycerin) release nitric oxide, dilating blood vessels, and are used in treatment of chronic and acute cardiac chest pain. Prescription of sildenafil and any nitrate medication is contraindicated because of the potential for a dangerous drop in blood pressure.

The 1999 National Ambulatory Medical Care Survey indicated that non-urologists were the chief medical practitioners prescribing sildenafil (71.8%), with urologists dispensing only 28.2% of prescriptions, Dr. Scales pointed out. However, it is unknown whether the co-prescribing in this study was done by urologists or non-urologists, he said.

Limitations of the study include the fact that there were no physician identifiers and no indication whether patients actually took the medications. Also, because this patient population had private insurance, they tended to be younger and have fewer chronic health conditions, so the data set may actually underestimate the true prevalence of this problem in the general population, Dr. Scales said.

Although PDE-5 inhibitors are safe and effective when used correctly, Dr. Scales said, careful review of medical history prior to prescribing them is indicated. There is also a need for integrated electronic systems to reduce potential prescription errors in the outpatient setting.