Sildenafil

Key points

• Patients should stop sildenafil and seek medical care if sudden loss of vision occurs in one or both eyes, which could be a sign of non-arteritic anterior ischaemic optic neuropathy; the label puts the annual incidence of NAION at 2.5 to 11.8 cases per 100,000 in males aged 50 and over.
• Prolonged erection beyond 4 hours and priapism have been reported infrequently, and the label states that if priapism is not treated immediately, penile tissue damage and permanent loss of potency could result.
• Sildenafil’s mean absolute bioavailability is 41%, with a range of 25 to 63%.
• It is cleared predominantly by the CYP3A4 hepatic pathway, with CYP2C9 as a minor route, so strong CYP3A4 inhibitors raise its plasma levels.

How it works

The UK summary of product characteristics sets out the sequence. Erection involves the release of nitric oxide in the corpus cavernosum during sexual stimulation. Nitric oxide activates guanylate cyclase, which raises levels of cyclic guanosine monophosphate, producing smooth muscle relaxation and allowing blood to flow in. Sildenafil is a potent and selective inhibitor of the cGMP-specific phosphodiesterase type 5 that degrades cGMP in that tissue. The same document is explicit that this restores impaired erectile function in the natural setting, meaning with sexual stimulation.

Safety specific to sildenafil

Nitrates. Sildenafil was shown to potentiate the hypotensive effect of nitrates, and giving it to anyone using organic nitrates or nitrites in any form, regularly or intermittently, is contraindicated. The label also says not to use sildenafil in patients taking a guanylate cyclase stimulator such as riociguat.

Sudden vision loss. Patients should stop sildenafil and seek medical care if a sudden loss of vision occurs in one or both eyes, which could be a sign of non-arteritic anterior ischaemic optic neuropathy. The label advises caution in anyone with a history of NAION, and notes that patients with a “crowded” optic disc may also be at increased risk.

Sudden hearing loss. Patients should stop taking PDE5 inhibitors including sildenafil and seek prompt medical attention in the event of sudden decrease or loss of hearing. The label records that these events may be accompanied by tinnitus and dizziness, and is careful to say it is not possible to determine whether they relate directly to the medicine or to other factors.

Priapism. Prolonged erection greater than 4 hours and priapism, meaning painful erections greater than 6 hours, have been reported infrequently. An erection persisting longer than 4 hours needs immediate medical assistance, because untreated priapism can cause penile tissue damage and permanent loss of potency. The label advises caution in men with anatomical deformation of the penis, or with conditions that may predispose to priapism such as sickle cell anaemia, multiple myeloma or leukaemia.

Alpha-blockers. Sildenafil and alpha-adrenergic blocking agents are both vasodilators with blood-pressure-lowering effects, and used together the effect on blood pressure can be additive. In some patients that combination lowers blood pressure enough to cause symptomatic hypotension: dizziness, lightheadedness or fainting. The label asks that patients be stable on alpha-blocker therapy before sildenafil is started.

The most common adverse reactions reported in clinical trials, at 2% or more, are headache, flushing, dyspepsia, abnormal vision, nasal congestion, back pain, myalgia, nausea, dizziness and rash.

When to seek urgent care

An erection lasting more than 4 hours, a sudden loss of vision in one or both eyes, and a sudden decrease or loss of hearing are the three events the label tells patients to stop the medicine and seek care for rather than wait out.

Sildenafil brand names in the UAE

The Emirates Drug Establishment’s drug directory records VIAGRA 50 as a tablet whose ingredient is sildenafil citrate equivalent to sildenafil, which is the name many readers in the region will know this medicine by.

Further reading

• VIAGRA- sildenafil citrate tablet, film coated
• Viagra 50 mg film-coated tablets
• VIAGRA ® 50 - Drug Directory