Phosphodiesterase 5 Inhibitors

Core Concepts

The physiology of erection: NO → cGMP → relaxation

Erection is a vascular event. Sexual stimulation activates the parasympathetic system, which triggers release of nitric oxide (NO) from the endothelium of the corpora cavernosa. NO diffuses into the smooth muscle cells and activates guanylate cyclase, which produces cGMP. cGMP relaxes the smooth muscle, the erectile tissue (corpora cavernosa) fills with blood, veins are compressed, and the penis becomes rigid. The signal is terminated by phosphodiesterase 5, the enzyme that degrades cGMP; when cGMP falls, the smooth muscle contracts again and the tissue returns to flaccidity. This single pathway explains the entire drug class.

What PDE5 inhibitors do

PDE5 inhibitors block the enzyme that destroys cGMP. With PDE5 inhibited, cGMP accumulates during sexual stimulation, smooth muscle relaxation is stronger and longer-lasting, and erection is enhanced. Three consequences follow logically:

• They require sexual stimulation. No NO, no cGMP to protect — so these drugs do not cause spontaneous erections and are not "aphrodisiacs"; they do not increase libido.
• They affect other smooth muscle too. PDE5 exists elsewhere, which explains class-typical adverse effects: headache, facial flushing, nasal congestion, and dyspepsia.
• Members differ in practical ways. Onset, duration, effect of food, and metabolism vary among the class members (for example, some act much longer than others, and food can alter absorption of some). Product-specific differences must be verified against current labeling — never assumed.

Beyond ED: pulmonary arterial hypertension

PDE5 is also present in pulmonary vascular smooth muscle. Inhibiting it there relaxes pulmonary vessels, lowering pulmonary vascular resistance — the rationale for using some PDE5 inhibitors to treat PAH. The same drug, a different indication, and typically a different dosing regimen: this is a reminder that indication determines how a drug is used, and that prescribing is always indication-specific and must be verified.

The nitrate contraindication

Nitrates — nitroglycerin in its various forms (sublingual tablets, sprays, patches, pastes) and isosorbide products — work by donating NO, which raises cGMP. Combining a nitrate with a PDE5 inhibitor means two mechanisms pushing cGMP up at once: vasodilation becomes profound and hypotension can be life-threatening. This is why the combination is contraindicated in current labeling, and why the nurse must ask about "as-needed" chest pain medications during every medication review — patients often do not volunteer them. When a patient with chest pain who has taken a PDE5 inhibitor needs emergency care, the health care team must be told about the PDE5 inhibitor so treatment decisions can account for it.

Adverse effects and drug interactions

Class-typical effects are extensions of vasodilation: headache, flushing, dyspepsia, nasal congestion, and dizziness. Visual changes (a blue tinge to vision) can occur with sildenafil because it also inhibits PDE6 in the retina; the effect is transient, but any vision change should be reported. Priapism — an erection lasting more than about four hours — is a rare but urgent adverse event because prolonged erection can damage erectile tissue; patients must know to seek emergency care immediately. Rare events such as sudden hearing loss have been reported and warrant flagging for review rather than dismissal.

Drug interactions beyond nitrates deserve attention: CYP3A4 inhibitors (some azole antifungals, some macrolides, and grapefruit juice) can raise PDE5 inhibitor levels, and combining PDE5 inhibitors with alpha blockers (Topic 8) requires caution per current guidance. Always verify the full medication list and current labeling.

Nursing considerations

The nursing role spans medication reconciliation (nitrates!), cardiac risk awareness, teaching, and respectful communication about sexual health:

• Ask about all erectile dysfunction products — prescribed or obtained otherwise — during every admission and medication review.
• Confirm nitrate use in any form before a PDE5 inhibitor is considered, and escalate concerns to the prescriber and pharmacist.
• Teach realistic expectations (stimulation required, not an aphrodisiac) and the priapism emergency rule.
• Document sexual-health conversations with person-first, nonjudgmental language; scope and documentation requirements vary by institution.

Common Confusions