Second-Line Erectile Dysfunction Therapy and the Priapism Emergency

First, prove the first-line drug actually failed

Most "PDE5 failures" are not failures of the drug — they are failures of use. Before escalating, confirm the PDE5 inhibitor was given a fair trial. Four questions do most of the work. Was the dose adequate — titrated up to the maximum, not left at a starter dose? Was there sexual stimulation — these drugs amplify the nitric-oxide signal but do not create it, so an erection will not appear without arousal? Were there enough attempts — sildenafil in particular should be tried up to six to eight times before it is called a failure? And was timing and food respected — sildenafil is best on an empty stomach because a fatty meal blunts and delays absorption, whereas tadalafil is far less food-dependent and gives a long window? This is a direct continuation of the PDE5-inhibitor chapter, where the cGMP mechanism is taught in full; here we simply hold that first-line therapy to account before abandoning it.

Then hunt the reversible cause — especially the drugs

Erectile dysfunction is a symptom, not a diagnosis, and a striking share of it is iatrogenic — caused by the patient's own medication list. The classic offenders read like a cardiology and psychiatry round: thiazide diuretics and beta-blockers (a very common cause in treated hypertensives), SSRIs and other antidepressants, antipsychotics, and finasteride and other 5-alpha-reductase inhibitors, which lower DHT and can dull libido and erection. This links straight to the Cardiovascular and Central Nervous System sections, where these same drugs are prescribed for their primary indications — the prescriber must weigh the sexual cost. Beyond drugs, comb for the modifiable: smoking (an endothelial poison that mirrors coronary disease — ED is often the first warning of vascular trouble), excess alcohol, hypogonadism (low testosterone), and depression, which is both a cause of ED and a consequence of it. Fixing a reversible cause can restore function without any second-line drug at all.

Second-line: alprostadil, the NO-independent route

Alprostadil is prostaglandin E1 — and its whole value is that it does not need nitric oxide. The dominant second-line drug is alprostadil, a synthetic prostaglandin E1. Instead of amplifying the NO–cGMP pathway that PDE5 inhibitors depend on, it binds prostaglandin receptors on cavernosal smooth muscle and raises cyclic AMP (cAMP), which relaxes the muscle and lets blood fill the corpora — an entirely separate second-messenger route. Because it acts downstream of the nerves, it produces an erection even when NO release has failed, which is exactly why it rescues men with diabetic or post-surgical nerve damage. It is delivered two ways: intracavernosal injection, where the man injects a tiny dose into the side of the shaft with a fine needle, or an intraurethral pellet (MUSE) inserted into the urethra, which is needle-free but less reliable. The characteristic adverse effects follow from the mechanism and the route: penile pain (prostaglandins are algogenic), and with repeated injection a risk of fibrosis or plaque at the injection site — and, above all, the drug can produce a prolonged erection or frank priapism, the emergency that dominates the second half of this chapter. The prostaglandin mechanism ties back to the Inflammation section, where prostaglandin E1 and its cAMP-linked receptors are taught in full.

The flip side: priapism, a urological emergency

An erection that outstays four hours is no longer an erection — it is ischaemia. Priapism is a persistent erection lasting more than four hours, unrelated to (or persisting after) sexual stimulation. The common and dangerous form is ischaemic (low-flow) priapism: blood becomes trapped in the corpora, cannot drain, and the tissue is starved of oxygen. Trapped deoxygenated blood makes the penis rigid and painful — and after only a few hours that stagnant, hypoxic, acidotic environment begins to kill the smooth muscle, so untreated ischaemic priapism scars the corpora and causes permanent fibrosis and impotence. That is why it is a true emergency measured in hours, not days. The causes span this very chapter and several others: intracavernosal injection therapy (alprostadil, and especially trimix, the leading iatrogenic cause), PDE5 inhibitors, sickle-cell disease (where sickled cells sludge and block outflow — a link to the Haematology section), and a range of drugs — notably the antipsychotic-adjacent trazodone and chlorpromazine, whose alpha-blocking action prevents the penis from de-tumescing, tying back to the Central Nervous System section.

The management of ischaemic priapism is a small masterpiece of applied pharmacology, and it inverts the second-line drug's own logic. First, the trapped blood is aspirated from the corpora with a needle to decompress and re-oxygenate the tissue. Then — the pharmacological core — a diluted sympathomimetic alpha-agonist, phenylephrine, is injected directly into the corpora. Where the erection was produced by relaxing cavernosal smooth muscle, phenylephrine does the opposite: as a selective alpha-1 agonist it constricts the smooth muscle and the penile arterial inflow, driving detumescence. Phenylephrine is chosen deliberately for its alpha-1 selectivity and minimal beta activity, to limit cardiovascular spill-over, though pulse and blood pressure are still monitored. This is the same alpha-adrenergic pharmacology taught in the Autonomic section — here turned to an unusual bedside use. If sickle-cell disease is the trigger, its systemic treatment (hydration, oxygen, analgesia, and haematology input) runs in parallel, because the priapism will keep recurring until the sickling is controlled.