PDE5 Inhibitors: The NO–cGMP Pathway and the Nitrate Trap
Few drugs tell a cleaner story about signal transduction than the ones that treat erectile dysfunction. An erection is not a plumbing event so much as a chemical cascade — a whisper of nitric oxide that ends in a wave of smooth-muscle relaxation. The famous "little blue pill" does not start that cascade; it simply refuses to let the body switch it off. That single, elegant idea explains everything: why the drug only works with arousal, why it is first-line for erectile dysfunction, why the same pathway treats pulmonary hypertension — and why, combined with a nitrate, it can kill.
A 58-year-old man mentions, almost as an afterthought at the end of a diabetes review, that his erections have failed for the past year. He is embarrassed; the doctor is not, because this is a common, treatable problem — and a warning sign. His blood pressure is high, he smokes, his cholesterol is borderline. The doctor knows that erectile dysfunction is often the first crack in the vasculature to show, arriving years before a heart attack. Before reaching for a prescription she asks one question that matters more than any other: does he take anything for chest pain? A nitrate spray, a patch, an old bottle of tablets? Because the tidy little pill she is about to offer, taken with a nitrate, could drop his blood pressure to the floor.
The physiology: an erection is a chemical signal
Follow the molecule and the whole pharmacology falls into place. Sexual stimulation — through nerves and the endothelium of the penis — releases nitric oxide (NO) into the smooth muscle of the corpus cavernosum. NO is a gaseous second messenger; it slips into the muscle cell and activates the enzyme guanylate cyclase. That enzyme manufactures cyclic GMP (cGMP), and cGMP is the actual signal for relaxation: it lowers intracellular calcium and makes the smooth muscle go slack. When the muscle in the cavernosal arteries and sinuses relaxes, blood rushes in; the swelling sinusoids compress the draining veins, trapping the blood, and the penis becomes rigid. The erection lasts exactly as long as cGMP stays high. This NO → guanylate cyclase → cGMP cascade is the same second-messenger logic taught in the Autonomic and Principles chapters — worth reviewing there, because once you own it, every drug below is obvious.
Every signal needs an off-switch, and here it is an enzyme: phosphodiesterase type 5 (PDE5). PDE5 is abundant in the penile smooth muscle, and its single job is to break cGMP down into inactive GMP. As soon as arousal fades and NO stops flowing, PDE5 mops up the remaining cGMP, calcium returns, the muscle contracts, the arteries narrow, and the erection subsides. PDE5 is the brake pedal on the whole system. Hold that image — the entire drug class is built on jamming that one brake.
The drugs: block the brake, don't press the accelerator
A PDE5 inhibitor amplifies a signal the body must still start on its own. The PDE5 inhibitors — sildenafil, tadalafil, vardenafil and avanafil — do exactly one thing: they block PDE5, so cGMP is no longer broken down. With the brake jammed, whatever cGMP arousal produces now accumulates and lingers, giving a firmer, longer-lasting erection. But note the crucial limit: these drugs do not create cGMP and they do not release NO. If there is no sexual stimulation, there is no NO, no cGMP to preserve, and no effect. They amplify a signal; they cannot initiate one. This is why they are taken in anticipation of, not instead of, arousal — and why they are useless as a party trick. They are first-line pharmacological therapy for erectile dysfunction of nearly any cause: vascular, diabetic, neurogenic, or psychogenic.
Think of cGMP as water filling a bath, and the erection as the rising water level. NO from arousal is the tap; PDE5 is the plughole, constantly draining the water away. A PDE5 inhibitor doesn't turn the tap harder — it plugs the drain. If the tap is running, the bath now fills higher and stays full for longer. But if nobody opens the tap, plugging an empty drain does nothing at all. That is the one-line summary of the entire class: block the drain, but you still need someone to open the tap.
The agents differ mainly in timing. Sildenafil and vardenafil are short-acting: taken about an hour before sexual activity, lasting roughly four to six hours, and their absorption is blunted by a heavy fatty meal (advise an empty stomach). Tadalafil is the outlier — its long half-life gives up to 36 hours of cover, earning it the nickname the "weekend pill" and freeing intimacy from the clock. That same long action lets tadalafil be given as a low daily dose, and at that dose it is separately licensed to treat the lower-urinary-tract symptoms of benign prostatic hyperplasia (BPH) — a neat two-for-one covered in the BPH chapter. Avanafil is the newest, valued for a fast onset.
- Erection = NO → guanylate cyclase → cGMP → smooth-muscle relaxation → arterial inflow.
- PDE5 is the enzyme that ends the signal by breaking cGMP down to GMP.
- PDE5 inhibitors block that breakdown so cGMP accumulates — amplifying, never initiating.
- They work ONLY with sexual stimulation — no arousal, no NO, no effect.
- Sildenafil/vardenafil short-acting (fatty food matters); tadalafil long-acting "weekend pill".
- Low-dose daily tadalafil is also licensed for BPH; the pathway also treats pulmonary hypertension.
The cardinal contraindication: nitrates
Two drugs that raise cGMP by different doors add up to a floor-level blood pressure. This is the single most important — and most examined — fact about the class. Nitrates (glyceryl trinitrate, isosorbide mononitrate and dinitrate) are NO donors: they treat angina by releasing nitric oxide, which raises cGMP in vascular smooth muscle everywhere and dilates blood vessels. So a nitrate floods the body with cGMP, and a PDE5 inhibitor stops that cGMP from being cleared. Give both, and cGMP soars systemically: profound, unopposed vasodilation and a catastrophic, potentially fatal drop in blood pressure. PDE5 inhibitors are therefore absolutely contraindicated with any nitrate, in any form. The same warning extends to nicorandil (a potassium-channel opener with a nitrate moiety, from the Cardiovascular chapter) and — a favourite hidden trap in exams and emergency departments — recreational amyl nitrite "poppers," which are pure NO donors. Cross-reference the Cardiovascular section on nitrates and the Toxicology section on poppers: if a man with chest pain has taken a PDE5 inhibitor in the last 24 hours (48 for tadalafil), you cannot safely give him a nitrate.
Erectile dysfunction is not just a quality-of-life issue — it is a cardiovascular risk marker. The penile arteries are small; they silt up and misbehave earlier than the coronaries, so ED often announces endothelial disease years before angina or infarction. Every new presentation is therefore a chance to screen: check blood pressure, glucose, lipids, and smoking. And crucially, before prescribing, assess whether the patient is fit enough for sexual activity at all — because prescribing a PDE5 inhibitor to a man who cannot survive the exertion, or who is on a nitrate, converts a treatment into a hazard.
Alpha-blockers, cardiovascular caution, and the side effects
Nitrates are the absolute bar; alpha-blockers are the relative one. Alpha-1 blockers (tamsulosin, doxazosin — used for BPH and hypertension) also lower blood pressure, and combined with a PDE5 inhibitor the drops can be additive, causing symptomatic postural hypotension. The rule is not "never" but "carefully": separate the dosing, start low, and warn the patient. Beyond that, caution — not necessarily prohibition — applies in significant cardiovascular disease, recent stroke or myocardial infarction, and severe hepatic or renal impairment, where dosing is reduced. The rest of the adverse-effect profile flows straight from the pharmacology: PDE5 is not confined to the penis, and neither is the drug.
The common effects are vasodilatory and predictable: headache, facial flushing, nasal congestion, and dyspepsia (relaxing the lower oesophageal sphincter). More distinctive is transient blue-tinged vision, most associated with sildenafil, because at higher doses it also inhibits PDE6 — an enzyme found in the retina. That PDE5/PDE6 cross-reactivity ties this chapter to the Ophthalmology section. Rare but serious ocular and otological events also occur: non-arteritic anterior ischaemic optic neuropathy (NAION), presenting as sudden painless visual loss, and sudden sensorineural hearing loss — both warrant stopping the drug. And the emergency every prescriber must recognise: priapism, a persistent erection lasting over four hours, which is a urological emergency because prolonged ischaemia can permanently damage the tissue. Finally, note the licensed cousins: the very same NO–cGMP pathway is exploited to treat pulmonary arterial hypertension, where sildenafil and tadalafil relax the pulmonary vasculature (cross-linked in the Cardiovascular chapter). These drugs are not licensed or effective for female sexual dysfunction.
Sildenafil (Viagra): short-acting, take ~1 h before activity, empty stomach; the agent most linked to blue vision (PDE6). Vardenafil (Levitra): short-acting, similar profile. Avanafil (Spedra): fast onset. Tadalafil (Cialis): long-acting up to 36 h ("weekend pill"), food-independent, and available as a low daily dose that also treats BPH. Whole-class rules: taken only with sexual stimulation; absolutely contraindicated with any nitrate or nicorandil; use caution and separate dosing with alpha-blockers; the same pathway is licensed for pulmonary arterial hypertension.
- ABSOLUTE contraindication: any nitrate or nicorandil — both raise cGMP → fatal hypotension.
- Recreational amyl nitrite "poppers" are NO donors — the same lethal interaction.
- Alpha-blockers: additive hypotension — use caution and separate the dosing.
- Common effects: headache, flushing, nasal congestion, dyspepsia; sildenafil → blue vision (PDE6).
- Serious/rare: NAION (sudden visual loss), sudden hearing loss, priapism (>4 h = urological emergency).
- ED is a cardiovascular risk marker — assess fitness for sex and screen for risk factors.
- Prescribing a PDE5 inhibitor to a man on any nitrate (or nicorandil, or who uses poppers) — this is the lethal, absolutely contraindicated combination.
- Expecting the drug to "cause" an erection on its own. Without sexual stimulation there is no NO and no effect — it amplifies, it does not initiate.
- Treating ED as a purely local problem and skipping the cardiovascular work-up — missing the endothelial disease and the man's fitness for sexual activity.
A 62-year-old man with stable angina asks his GP for sildenafil for erectile dysfunction. He uses a glyceryl trinitrate spray for chest pain. What is the most appropriate action?
- An erection is a signal cascade: sexual stimulation → NO → guanylate cyclase → cGMP → smooth-muscle relaxation and penile arterial inflow; PDE5 ends it by degrading cGMP.
- PDE5 inhibitors (sildenafil, tadalafil, vardenafil, avanafil) block that breakdown so cGMP accumulates — amplifying the erection but only WITH sexual stimulation; they are first-line for ED.
- ABSOLUTELY contraindicated with any nitrate/nicorandil/poppers (additive cGMP → fatal hypotension); use caution with alpha-blockers; ED is itself a cardiovascular risk marker.
- Side effects: headache, flushing, nasal congestion, dyspepsia, blue vision (PDE6), rare NAION, hearing loss and priapism; tadalafil daily also treats BPH and the pathway treats pulmonary hypertension — not for women.
- Rang & Dale's Pharmacology — The reproductive system and drugs used in erectile dysfunction.
- Katzung — Basic & Clinical Pharmacology: Nitric oxide, cGMP and PDE5 inhibitors.
- British National Formulary (BNF) — Sildenafil, tadalafil, vardenafil, avanafil: interactions and contraindications with nitrates.
- NICE Clinical Knowledge Summaries — Erectile dysfunction: assessment and management.
- European Association of Urology (EAU) Guidelines on Sexual and Reproductive Health — Male sexual dysfunction.
- Galiè N, et al. Sildenafil citrate therapy for pulmonary arterial hypertension (SUPER-1). New England Journal of Medicine.
