Sildenafil citrate (Viagra) is a specific phosphodiesterase type V inhibitor with a selective inhibitory effect on cyclic guanosine monophosphate (cGMP). Its enhancement of nitric oxide release leads to an increase in cGMP concentrations, which is responsible for its main clinical effect—relaxation of the smooth muscle in the corpus cavernosum. The increase in blood flow into the penis helps to alleviate problems of erectile dysfunction.1

Minor side effects—such as headache, flushing, nasal congestion, and defects in colour vision—and serious cardiovascular and cerebrovascular effects, have been described.1 Convulsive seizures have been an unknown neurological side effect until now. We report two cases of patients who first had a seizure soon after using sildenafil.

Case 1—A 63 year old man with a history of hypertension, but who was otherwise healthy, was admitted to hospital because of a first episode of generalised tonic-clonic seizures (GTCS). He had been prescribed sildenafil (50 mg) as required for sexual dysfunction. Three hours after taking sildenafil for the first time, he had a typical tonic-clonic seizure, as observed by his spouse. He was awake when he was admitted, and results of neurological examination, brain computed tomography, magnetic resonance imaging, and electroencephalography were normal. Electrocardiography, stress electrocardiography, echocardiography and cardiac scan with dipyridamole test, as well as carotid Doppler ultrasonography, showed no abnormalities. Two days later, he was discharged from the hospital with the recommendation that he stop using sildenafil. However, three months later, he decided on his own responsibility to try the drug again, and four hours later, had a tonic-clonic seizure. He was then prescribed carbamazepine (600 mg/day). He has been free of seizures for two years.

Case 2—An otherwise healthy 54 year old man was admitted to our department after having a first tonic-clonic seizure about 4.5 hours after he had taken sildenafil for the first time. His neurological examination, brain computed tomography, magnetic resonance imaging, and electroencephalography with sleep deprivation were normal.