Mechanism of action
Tadalafil selectively inhibits phosphodiesterase type 5 (PDE5), the enzyme responsible for degrading cyclic guanosine monophosphate (cGMP) in smooth muscle cells. Sexual stimulation triggers nitric oxide (NO) release from endothelial cells, which activates guanylate cyclase to produce cGMP. cGMP causes smooth muscle relaxation in the corpus cavernosum, allowing increased blood flow and penile erection.
By inhibiting PDE5, tadalafil prevents cGMP breakdown and prolongs its vasodilatory effects. This mechanism is not limited to penile tissue—PDE5 is also abundant in pulmonary vascular smooth muscle, which explains tadalafil's efficacy in pulmonary arterial hypertension. The drug has minimal effect on other phosphodiesterase isoforms at therapeutic doses, though some cross-reactivity with PDE11 (skeletal muscle, testes) and PDE6 (retina) occurs at higher concentrations.
Tadalafil does not directly cause erections; sexual stimulation and intact NO-cGMP signaling are required. It simply amplifies the physiologic erectile response when NO is released.
- Tadalafil's half-life is 17.5 hours, the longest among PDE5 inhibitors, enabling once-daily dosing.
- Absolute contraindication with nitrates due to potentially fatal synergistic hypotension via cGMP elevation.
- Mechanism: selective PDE5 inhibition increases cGMP, causing smooth muscle relaxation in corpus cavernosum and pulmonary vasculature.
- Approved for erectile dysfunction, pulmonary arterial hypertension, and benign prostatic hyperplasia.
- Back pain and myalgia are more common with tadalafil than other PDE5 inhibitors, possibly due to PDE11 cross-reactivity.
- Metabolized by CYP3A4; dose reduction required with potent inhibitors like ketoconazole or ritonavir.
- Priapism (erection >4 hours) is a medical emergency; higher risk in sickle cell disease, leukemia, multiple myeloma.
- NAION (sudden vision loss) is a rare but serious adverse effect requiring immediate discontinuation.
Indications
Erectile dysfunction
Tadalafil is FDA-approved for erectile dysfunction (ED) in adult men. It can be used on-demand (taken before anticipated sexual activity) or as a daily low-dose regimen. The long half-life makes it particularly suited for daily dosing, providing continuous therapeutic coverage.
Pulmonary arterial hypertension
Tadalafil is approved for pulmonary arterial hypertension (PAH) to improve exercise capacity. By reducing pulmonary vascular resistance through smooth muscle relaxation, it decreases right ventricular afterload. It is used in WHO Group 1 PAH and is particularly useful in patients who cannot tolerate prostacyclin analogs or endothelin receptor antagonists.
Benign prostatic hyperplasia
Tadalafil is approved for the signs and symptoms of benign prostatic hyperplasia (BPH), either alone or in combination with finasteride. The mechanism involves relaxation of smooth muscle in the prostate and bladder neck, improving urinary flow. PDE5 inhibitors do not reduce prostate size.
Combination indication
Tadalafil is also approved for the combined treatment of ED and BPH in men with both conditions.
Pharmacokinetics
Absorption: Tadalafil is rapidly absorbed after oral administration, with peak plasma concentrations reached in approximately 2 hours. Food does not significantly affect bioavailability, so it can be taken without regard to meals.
Distribution: The drug is highly protein-bound (94%), primarily to albumin. Volume of distribution is approximately 63 liters.
Metabolism: Tadalafil is metabolized predominantly by CYP3A4 in the liver. The major circulating metabolite (methylcatechol glucuronide) is inactive. A minor pathway involves CYP3A5.
Elimination: The mean terminal half-life is 17.5 hours in healthy subjects, which is significantly longer than sildenafil (4 hours) or vardenafil (4-5 hours). This long half-life enables once-daily dosing and provides a wider window for sexual activity. Tadalafil is excreted predominantly as metabolites in feces (61%) and urine (36%).
Special populations: Clearance is reduced in patients with hepatic or renal impairment, necessitating dose adjustments. In patients with creatinine clearance <30 mL/min or on hemodialysis, maximum recommended dose is lower. Elderly patients may have reduced clearance but dose adjustment based solely on age is not required.
Adverse effects
Common adverse effects
- Headache (11-15%): the most common side effect, related to cerebral vasodilation
- Dyspepsia (10-12%): secondary to relaxation of the lower esophageal sphincter
- Back pain and myalgia (up to 6%): may be related to PDE11 inhibition in skeletal muscle; typically delayed 12-24 hours after dosing
- Flushing (3-4%): cutaneous vasodilation
- Nasal congestion (4%): nasal mucosal vasodilation
Serious adverse effects
Hypotension: Tadalafil potentiates the hypotensive effects of nitrates through additive vasodilation. The combination can cause severe, potentially fatal hypotension and is an absolute contraindication. Hypotension may also occur with alpha-blockers, though cautious co-administration is possible with dose adjustment.
Priapism: Prolonged erection lasting more than 4 hours is a medical emergency requiring immediate intervention to prevent permanent erectile dysfunction. Risk is higher in patients with sickle cell disease, multiple myeloma, or leukemia.
Visual disturbances: Non-arteritic anterior ischemic optic neuropathy (NAION) has been reported rarely. Patients with "crowded disc" anatomy may be at higher risk. Sudden vision loss requires immediate discontinuation. Transient blue-green color tinge or increased light sensitivity may occur due to PDE6 inhibition in the retina.
Hearing loss: Sudden sensorineural hearing loss has been reported with PDE5 inhibitors, sometimes accompanied by tinnitus and dizziness. The mechanism is unclear.
Cardiovascular events: Sexual activity itself carries cardiovascular risk in patients with heart disease. Exertional angina, recent MI, or unstable cardiovascular disease warrants careful evaluation before prescribing.
Contraindications and warnings
Absolute contraindications
- Concomitant nitrate use (nitroglycerin, isosorbide mononitrate/dinitrate, amyl nitrite): risk of severe hypotension. This contraindication extends to recreational "poppers."
- Concomitant guanylate cyclase stimulators (riociguat): risk of hypotension
- Known hypersensitivity to tadalafil
Conditions where use is not recommended
The following conditions were exclusion criteria in clinical trials, and tadalafil use is not recommended in these populations:
- Recent stroke or myocardial infarction (within 90 days)
- Unstable angina or angina during sexual intercourse
- NYHA Class II or greater heart failure in the past 6 months
- Severe hypotension (BP <90/50 mmHg)
- Uncontrolled arrhythmias or severe hypertension (BP >170/100 mmHg)
Warnings and precautions
Anatomic penile deformation: Patients with Peyronie's disease, angulation, or cavernosal fibrosis may be predisposed to priapism.
Bleeding disorders: PDE5 inhibitors may potentiate antiplatelet effects of nitric oxide, though clinical significance is unclear. Use caution in patients with active peptic ulceration.
Retinitis pigmentosa: These patients have impaired PDE6 function; PDE5 inhibitors may worsen vision.
Alpha-blocker co-administration: Orthostatic hypotension may occur. Initiate tadalafil at the lowest dose if patient is stable on alpha-blocker therapy, or separate dosing by several hours.
Pulmonary veno-occlusive disease: PDE5 inhibitors may precipitate pulmonary edema in these patients.
Pregnancy and lactation: Tadalafil is not indicated for use in women. In PAH, it is pregnancy category B but generally avoided due to limited data.
Drug interactions
CYP3A4 inhibitors
Potent CYP3A4 inhibitors (ketoconazole, itraconazole, ritonavir, clarithromycin) significantly increase tadalafil plasma concentrations. Dose reduction is required. Grapefruit juice also inhibits CYP3A4 and should be avoided.
CYP3A4 inducers
Rifampin, phenytoin, carbamazepine, and other CYP3A4 inducers may reduce tadalafil efficacy by increasing its metabolism.
Nitrates
Absolute contraindication due to potentially fatal hypotension. The long half-life of tadalafil means nitrates cannot be safely administered for at least 48 hours after the last dose.
Alpha-blockers
Doxazosin, terazosin, tamsulosin, and alfuzosin can cause additive hypotensive effects. When co-prescribing, use the lowest effective tadalafil dose and consider separating administration times. Patients should be stable on alpha-blocker therapy before adding tadalafil.
Antihypertensives
Tadalafil may potentiate the blood pressure-lowering effects of antihypertensives, particularly in patients on multiple agents.
Other PDE5 inhibitors
Concomitant use of multiple PDE5 inhibitors is not recommended and does not provide additional benefit.
Riociguat
This soluble guanylate cyclase stimulator is contraindicated with PDE5 inhibitors due to risk of hypotension.
Clinical pearls
- Tadalafil's 17.5-hour half-life is the longest among PDE5 inhibitors, allowing once-daily dosing and a 36-hour window of responsiveness (hence the "weekend pill" nickname).
- Timing flexibility: Unlike sildenafil, tadalafil's long duration means patients do not need to time intercourse closely to dosing, reducing performance anxiety.
- Food has no effect on absorption, unlike sildenafil, which is delayed by high-fat meals.
- Nitrate washout: Because of its long half-life, nitrates must be avoided for at least 48 hours after tadalafil, versus 24 hours for shorter-acting PDE5 inhibitors.
- Back pain and myalgia are more common with tadalafil than other PDE5 inhibitors, possibly due to PDE11 inhibition. Symptoms are typically delayed and self-limited.
- PAH indication uses daily dosing, not on-demand dosing. The formulation and dosing for PAH differ from those for ED.
- BPH mechanism: PDE5 is present in bladder and prostate smooth muscle; inhibition improves lower urinary tract symptoms independent of erectile function.
- Exam tip: If a vignette describes a patient taking nitroglycerin for angina who presents with syncope after starting a new medication for ED, think tadalafil (or another PDE5 inhibitor). This combination is a classic contraindication.
- Vision changes: While rare, NAION is a reported adverse effect. Patients reporting sudden vision loss should stop the drug immediately and seek ophthalmologic evaluation.
- Does not cure ED: Tadalafil treats symptoms but does not address underlying causes such as diabetes, hypertension, or psychogenic factors. Comprehensive management includes treating comorbidities.
Practice questions
1. A 62-year-old man with a history of coronary artery disease presents to the emergency department with chest pain. He took sublingual nitroglycerin 30 minutes ago without relief. His wife reports he started taking a new medication for erectile dysfunction 18 hours ago. On examination, his blood pressure is 75/40 mmHg and heart rate is 115 bpm. ECG shows no acute ST changes. Which medication did he most likely start?
2. A 58-year-old man taking tadalafil daily for benign prostatic hyperplasia develops bilateral lower back pain and myalgia that began 18 hours after his morning dose. The pain is moderate and not associated with trauma, fever, or neurological symptoms. Physical examination shows normal muscle strength and reflexes. Which mechanism best explains his symptoms?
3. A 45-year-old man with pulmonary arterial hypertension controlled on tadalafil is prescribed ketoconazole for a fungal infection. His physician should:
Frequently asked questions
How does tadalafil work?
Tadalafil works by selectively inhibiting phosphodiesterase-5 (PDE5), the enzyme that breaks down cyclic GMP in smooth muscle cells. When sexual stimulation triggers nitric oxide release, cGMP accumulates and causes smooth muscle relaxation in the penis, allowing increased blood flow and erection. Tadalafil amplifies this natural response but does not cause erections without sexual stimulation. The same mechanism reduces pulmonary vascular resistance in patients with pulmonary arterial hypertension.
What are the side effects of tadalafil?
Common side effects include headache (most frequent), dyspepsia, back pain, myalgia, flushing, and nasal congestion. These result from vasodilation and smooth muscle relaxation. Serious adverse effects include severe hypotension (especially with nitrates), priapism (prolonged erection requiring emergency treatment), sudden vision loss from non-arteritic anterior ischemic optic neuropathy, and sudden hearing loss. Patients should seek immediate medical attention for erections lasting more than 4 hours or sudden changes in vision or hearing.
Can you take tadalafil with blood pressure medication?
Tadalafil can interact with blood pressure medications, particularly nitrates (absolute contraindication) and alpha-blockers. Nitrates combined with tadalafil cause potentially fatal hypotension and must never be used together. Alpha-blockers like doxazosin or tamsulosin can cause additive hypotensive effects; if co-prescribed, use the lowest tadalafil dose and ensure the patient is stable on the alpha-blocker first. Other antihypertensives may also have enhanced effects, so blood pressure should be monitored when starting tadalafil.
Is tadalafil a controlled substance?
No, tadalafil is not a controlled substance in the United States or most other countries. It is available by prescription only but is not classified under the DEA's controlled substance schedules. It has no abuse potential or risk of physical dependence. However, it requires medical supervision due to cardiovascular contraindications and potential drug interactions, particularly with nitrates and CYP3A4 inhibitors.
Tadalafil vs sildenafil — what is the difference?
The primary difference is duration of action: tadalafil has a 17.5-hour half-life (36-hour window of effectiveness) while sildenafil has a 4-hour half-life (4-6 hour window). This allows tadalafil to be taken daily or provides greater timing flexibility for on-demand use. Food does not affect tadalafil absorption but delays sildenafil. Tadalafil is also approved for benign prostatic hyperplasia and pulmonary arterial hypertension, while sildenafil is approved for ED and PAH. Back pain is more common with tadalafil; visual disturbances may be slightly more common with sildenafil due to greater PDE6 cross-reactivity.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: G04BE08
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.