Lay Summary: Evaluation of the safety of sildenafil (Viagra)

Background

Sildenafil (Viagra) is a medicine used to treat men with impotence (difficulty getting or keeping an erection). In 1999 the DSRU conducted a study to measure the frequency of known side effects of sildenafil and to identify any previously unknown side effects. The study also investigated concerns that sildenafil could have adverse effects on the heart and circulation. Since sexual intercourse without the use of sildenafil can put an increased strain on the heart, reports of such events are difficult to interpret.


Methods

The research method used is known as Prescription Event Monitoring (PEM). Patients prescribed sildenafil between April and August 1999 were identified from data on dispensed prescriptions supplied, in confidence, by the Prescription Pricing Authority (PPA), a processing centre to which pharmacists send dispensed prescriptions for payment purposes.

After approximately 18 months, GPs were asked to provide information on any medical events that patients experienced since sildenafil was first prescribed. A medical event was defined as anything that was important enough to have been recorded in the medical notes regardless of whether or not it was thought to be related to treatment. The information received was then analysed and used to measure the frequency of known side effects and identify previously unknown effects of sildenafil.

All prescription information received from the PPA and anonymised medical information received from GPs is treated in strict confidence by the DSRU, which is allowed under the Data Protection Act of 1998 to hold such information for research purposes.


Results

Information was received relating to 22,471 male patients and 2 female patients who had been prescribed sildenafil. The majority (73.8%) of patients were prescribed sildenafil to treat impotence. Other patients were prescribed sildenafil because they had diseases such as diabetes (high blood sugar levels) that is known to cause impotence.

There were 145 reports by GPs of adverse drug reactions to sildenafil. The most commonly reported adverse drug reactions were headache, flushing and indigestion, dizziness and disturbance of vision. Heart disease was the most commonly reported medical reason for stopping sildenafil, this occurred in 135 patients.

The most common causes of death during the study were cancer, heart attacks and heart disease. An in-depth analysis found that the number of deaths due to heart-related disease was not higher than the number of men expected to die from heart disease in the general population in England.


Conclusion

This study found that the most common side effects of sildenafil included headache, flushing, indigestion, dizziness and disturbance of vision. These are expected effects based on earlier studies. No unexpected effects were found and there was no evidence that sildenafil was associated an excess number of deaths due to heart disease.