Alprostadil (Caverject/MUSE) Overdose and Accidental Excess Dose

What Alprostadil Is and How It Works

Alprostadil is synthetic prostaglandin E1 (PGE1), a naturally occurring lipid mediator that relaxes smooth muscle through a cyclic AMP-dependent pathway. It is available in two delivery systems: Caverject (intracavernosal injection, 2.5 to 40 mcg) and MUSE (medicated urethral system for erection, 125 to 1,000 mcg suppository). Both are indicated for erectile dysfunction refractory to oral phosphodiesterase-5 inhibitors. The FDA prescribing information for Caverject lists priapism as the most clinically significant adverse event.

Mechanism at the Cellular Level

Alprostadil binds EP2 and EP3 receptors on cavernosal smooth muscle cells, activating adenylyl cyclase and raising intracellular cyclic AMP. This activates protein kinase A, which phosphorylates myosin light-chain kinase and reduces calcium sensitivity, causing relaxation of the trabecular smooth muscle and helicine arteries. Blood pools in the corpora cavernosa, compressing the emissary veins and producing erection. The NIH consensus on erectile dysfunction physiology confirms cyclic AMP as the primary second-messenger for non-adrenergic, non-cholinergic penile tumescence.

Pharmacokinetics Relevant to Overdose Risk

After intracavernosal injection, alprostadil is metabolized locally and in the lungs on first pass. Systemic bioavailability is low, roughly 80% of the drug is cleared pulmonarily within minutes. Peak plasma levels after a 20 mcg injection are below 3 pg/mL in most patients, according to data reviewed in the Caverject FDA label. MUSE delivers higher systemic exposure because urethral mucosa absorbs drug into the corpus spongiosum and peri-urethral vasculature before reaching the corpora, which is why cardiovascular side effects are proportionally more common with MUSE than with injection.

The Linet 1996 Trial: Efficacy Context That Explains Dosing Errors

The landmark randomized controlled trial by Linet and Ogrinc, published in the New England Journal of Medicine in 1996 (N=296), demonstrated that intracavernosal alprostadil produced erections sufficient for intercourse in approximately 70% of men with refractory ED across dose groups ranging from 2.5 mcg to 20 mcg. Adverse events in that trial included prolonged erection in 4.4% of subjects and penile pain in 29%. The trial used in-office titration to find the lowest effective dose, a protocol the FDA requires, yet real-world patients frequently self-escalate outside the titrated range.

The 4.4% prolonged erection rate seen in Linet et al. Occurred under supervised dosing. Self-administered doses above the titrated ceiling carry a substantially higher risk, though the exact incidence of out-of-office overdose-related priapism has not been quantified in a large prospective registry.

Defining an Alprostadil Overdose

Dose-Based Definition

No single dose of alprostadil constitutes an absolute overdose in the pharmacotoxicological sense. The functional overdose is any amount that produces an erection lasting more than 4 hours, regardless of whether that dose was within or above the labeled range. A 5 mcg injection can cause priapism in a highly sensitive patient; a 40 mcg injection may not in another. The American Urological Association (AUA) guideline on priapism defines ischemic priapism as a persistent, unwanted erection exceeding 4 hours, accompanied by cavernosal hypoxia and acidosis.

Ischemic vs. Non-Ischemic Priapism

Alprostadil almost exclusively causes ischemic (low-flow) priapism when it produces overdose-level erections. This is the dangerous type: arterial inflow continues but venous outflow is obstructed by compartment pressure. Cavernosal blood gas analysis shows PO2 <30 mmHg, PCO2 >60 mmHg, and pH <7.25 after several hours. These values, cited in the AUA priapism guidelines, mirror tissue ischemia equivalent to limb compartment syndrome.

Non-ischemic (high-flow) priapism does not occur from alprostadil overdose; it requires arteriovenous fistula formation, typically from perineal trauma.

Systemic Toxicity: A Separate Concern

Beyond priapism, very high doses or erratic absorption can produce systemic prostaglandin effects. These include hypotension (vasodilation), tachycardia or reflex bradycardia, dizziness, and syncope. MUSE carries a specific FDA black-box-adjacent warning about hypotension and syncope: the label recommends the first dose be administered in a medical office with blood pressure monitoring. The MUSE prescribing information notes syncope in 3.3% of patients in clinical trials, and hypotension in approximately 3%.

Recognizing an Alprostadil Overdose: Symptom Timeline

0 to 2 Hours

A therapeutic alprostadil erection typically subsides within 30 to 60 minutes after ejaculation or within 1 to 2 hours without sexual stimulation. An erection persisting beyond 90 minutes with no signs of detumescence should prompt the patient to call the prescribing provider. Pain is often absent in the first 1 to 2 hours because cavernosal oxygenation is still partially maintained.

2 to 4 Hours

Cavernosal ischemia accelerates. Pain becomes prominent, described as a dull, deep ache inside the penis. The shaft feels rigid but the glans remains soft, which distinguishes ischemic priapism from normal erection. The patient should go to an emergency department at this point, not wait for resolution. Aspiration and phenylephrine reversal, performed within 4 hours, carry a high success rate and low permanent injury rate, according to the AUA priapism management algorithm.

Beyond 6 Hours

Irreversible damage begins. Smooth muscle necrosis, collagen deposition, and fibrosis can develop within 12 to 24 hours of sustained ischemia. Erectile dysfunction following untreated priapism is common: one prospective series reported ED in over 90% of men whose priapism lasted more than 24 hours. Early intervention is the only protection against this outcome.

Emergency Management Protocol

The following stepwise protocol synthesizes recommendations from the AUA priapism guideline, the European Association of Urology (EAU) sexual medicine guidelines, and the FDA label for Caverject.

Step 1: Patient Self-Care (0 to 2 Hours, If Directed by Provider)

Some prescribers pre-authorize patients to apply ice packs to the perineum and inner thigh, and to walk or exercise briefly to promote adrenergic tone. These measures are adjuncts only. Oral pseudoephedrine 60 mg has been described in older literature but the AUA guideline does not endorse it as definitive therapy and notes limited evidence for its efficacy.

Step 2: Emergency Department Aspiration

The clinician inserts a 19-gauge butterfly needle into the lateral corpus cavernosum, aspirates 20 to 30 mL of dark, deoxygenated blood, and observes for detumescence. Cavernosal blood gas can be sent at this time to confirm ischemic status. Aspiration alone resolves approximately 30% of ischemic priapism episodes. If the erection recurs after aspiration, pharmacological reversal is required.

Step 3: Intracavernosal Phenylephrine

Phenylephrine 200 mcg is injected intracavernosally every 3 to 5 minutes, up to a total of 1,000 mcg over 1 hour, while cardiac monitoring is maintained. Phenylephrine is preferred over epinephrine because it is a selective alpha-1 agonist with minimal beta-adrenergic activity, reducing the risk of arrhythmia. The AUA guideline states that "phenylephrine should be the sympathomimetic agent of choice due to its cardiovascular safety profile." Success rates for aspiration plus phenylephrine exceed 80% when the procedure is performed within 6 hours of onset.

Step 4: Surgical Shunting

If pharmacological reversal fails after 60 minutes of phenylephrine, surgical or percutaneous shunting is required. Distal shunts (T-shunt, Al-Ghorab, Winter shunt) create a communication between the corpus cavernosum and corpus spongiosum. Proximal shunts are reserved for failure of distal procedures. The decision to escalate to surgery should not be delayed past the 8-hour mark.

Managing Systemic Toxicity From Alprostadil

Hypotension from alprostadil is treated with the same approach used for any vasodilatory hypotension: supine positioning, intravenous normal saline bolus (500 mL over 15 minutes), and vasopressors if blood pressure does not respond. Phenylephrine is a reasonable pressor choice here as well, since it directly counteracts prostaglandin-mediated vasodilation.

The MUSE prescribing information advises that patients with cardiovascular disease who use MUSE should have their first dose administered under medical supervision. Concurrent use of antihypertensives, nitrates, or other vasodilators multiplies the hypotensive risk. There is no specific antidote for systemic alprostadil toxicity beyond supportive care.

Bradycardia, when present, typically reflects a vagal reflex from pain rather than direct chronotropic depression by alprostadil. Atropine 0.5 mg IV is appropriate if symptomatic bradycardia accompanies hypotension.

Risk Factors That Raise Overdose Probability

Not every patient metabolizes or responds to alprostadil identically. Several factors increase the likelihood that a given dose will produce prolonged erection or systemic toxicity.

Patient-Level Factors

Sickle cell disease, sickle cell trait, and other hematologic disorders that impair blood rheology are associated with significantly higher priapism risk with any vasoactive drug. The AUA guideline lists hematologic dyscrasias as a major predisposing condition for recurrent ischemic priapism. Patients with these conditions should use the minimum effective dose, and providers may consider a lower starting dose than the standard 2.5 mcg.

Neurogenic erectile dysfunction, such as that following radical prostatectomy, is associated with cavernosal smooth muscle hypersensitivity to alprostadil. Post-prostatectomy patients often respond at doses 50% to 75% lower than the standard starting dose, a pattern well documented in penile rehabilitation literature.

Drug Interactions

Concurrent use of other vasoactive agents or drugs that prolong erection can compound overdose risk. These include:

• Phosphodiesterase-5 inhibitors (sildenafil, tadalafil, vardenafil): combination with alprostadil is not recommended by the FDA label.
• Anticoagulants: do not increase overdose risk per se but complicate any penile aspiration procedure by increasing bleeding risk.
• Antidepressants with alpha-blocking properties (trazodone): may potentiate cavernosal vasodilation.

Injection Technique Errors

Injecting into the corpus spongiosum rather than the corpus cavernosum delivers drug to a higher-flow compartment with different pharmacodynamics. Injecting both corpora with a full dose (instead of the midline crossing technique where one injection fills both corpora via the septal perforations) doubles the effective dose. Providers should verify injection technique at each in-office training visit.

Preventing Overdose: Titration Protocol and Patient Education

The FDA-required titration protocol for Caverject starts at 2.5 mcg, administered in-office, with the dose increased in 2.5 to 5 mcg increments at intervals of at least 24 hours between doses. The goal is to identify the lowest dose that produces a satisfactory erection lasting no more than 60 minutes. Patients who achieve erections lasting more than 60 minutes in the office are not candidates for that dose at home.

Patient education must cover four specific instructions:

1. Never exceed the in-office titrated dose at home.
2. Never inject more than once in 24 hours.
3. Limit use to a maximum of 3 injections per week to prevent fibrosis.
4. Go to an emergency department immediately if the erection lasts beyond 4 hours.

The Caverject FDA label recommends that these instructions be given verbally and in writing before the patient leaves the office.

Special Populations

Patients With Cardiovascular Disease

MUSE poses the greatest systemic risk in this group. A 2001 study in the International Journal of Impotence Research (N=511) found that MUSE-related hypotension was significantly more common in men on antihypertensive therapy. For these patients, intracavernosal alprostadil is generally preferred over MUSE because systemic absorption is lower, but blood pressure should still be checked 30 minutes after the first in-office injection.

Older Adults

Men over 65 have reduced alpha-adrenergic tone in cavernosal smooth muscle, meaning the drug-to-response curve is steeper and overdose erections occur at lower doses. Titration in this population should begin at 1.25 mcg if available, or 2.5 mcg with shorter in-office observation intervals.

Patients on Anticoagulation

Aspiration and phenylephrine injection can still be performed, but the clinician should apply compression for at least 5 minutes after needle withdrawal and check for hematoma formation. Warfarin reversal is not required for this procedure.

When to Call Poison Control vs. Go Directly to the ED

A patient who has injected or inserted excess alprostadil and has a sustained erection beyond 3 hours should go directly to an emergency department. Poison Control (1-800-222-1222 in the United States) can provide guidance for mild cases, systemic symptoms without priapism, or MUSE-related dizziness, but Poison Control cannot perform aspiration. The FDA MedWatch reporting system should be used by providers to report any serious adverse event, including priapism requiring surgical intervention.