Sildenafil

What it is

Sildenafil inhibits phosphodiesterase type 5, the enzyme that breaks down cGMP in vascular smooth muscle. Block it and the signal that relaxes those vessels persists, so blood flow into the corpus cavernosum is sustained rather than cut short.

Pfizer was not looking for this. The compound entered development in 1989 as an angina treatment, performed unremarkably on that endpoint, and produced a side effect that trial participants were notably reluctant to return unused tablets over. The pivotal erectile dysfunction data was published in the New England Journal of Medicine in May 1998, and approval followed in March of the same year.

It matters here because it is the compound the other two on this site are implicitly compared to, and because leaving it out would be an odd gap in a reference that covers tadalafil and vardenafil.

Why the timing works differently

The whole practical distinction between the three PDE5 inhibitors is duration, and sildenafil sits at the short end.

Roughly four hours of half-life means a defined window: take it, wait, and the effect is largely gone by the next morning. Tadalafil runs about seventeen hours, which is why it is the one people take daily at a low dose and stop thinking about. Vardenafil sits close to sildenafil on timing but with less of the visual effect.

None of the three is more effective than the others in any way that survives head to head comparison. They differ on how long they last, how food affects them, and which side effect you are most likely to notice. Choosing between them is a scheduling decision, not an efficacy one.

Practical notes

• Food is the usual culprit when it fails. A high fat meal delays peak concentration substantially. People conclude the drug does not work for them when what actually happened is that they took it after dinner.
• It does not create desire. PDE5 inhibitors act on blood flow and require arousal to do anything at all. If the problem is desire rather than mechanics, this is the wrong class entirely, and bremelanotide is the one that acts on the other half of the problem.
• The nitrate interaction is the one rule with no exceptions. This includes recreational nitrites, not only prescribed nitrates. It is the reason emergency clinicians ask about PDE5 use before treating chest pain.
• Erectile dysfunction is frequently the first sign of vascular disease. The arteries involved are small and narrow early. Treating the symptom without anyone looking at cardiovascular risk is a missed opportunity, and often a missed diagnosis.

Side effects and warning signs

Commonly reported

• Headache, the most common by a wide margin
• Facial flushing
• Dyspepsia and reflux
• Nasal congestion
• A blue tinge to vision or light sensitivity, from weak cross-inhibition of PDE6 in the retina

Stop and get medical help

• Any nitrate medication. The combination can drop blood pressure catastrophically and is an absolute contraindication, not a caution to weigh
• An erection lasting more than four hours. Priapism is a genuine emergency and delay costs tissue
• Sudden loss of vision or hearing in one eye
• Chest pain during activity after dosing, which needs assessment that cannot involve nitrates

Sources

1. Goldstein et al., Oral sildenafil in the treatment of erectile dysfunction, NEJM 1998trial
2. A systematic review of sildenafil mortality through the yearsreview

From the community

Discussion for this compound concentrates in r/PeptideAdvice.

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