Tadalafil works by selectively inhibiting the enzyme phosphodiesterase type 5 (PDE5), which is responsible for breaking down cyclic guanosine monophosphate (cGMP). cGMP is a key mediator of smooth muscle relaxation and blood vessel dilation. The mechanism of action involves:

1. Erectile Dysfunction:
   • During sexual stimulation, nitric oxide (NO) is released in the penile tissue, which activates the enzyme guanylate cyclase.
   • This increases the production of cGMP, leading to relaxation of smooth muscles in the penile arteries and increased blood flow to the corpus cavernosum, resulting in an erection.
   • By inhibiting PDE5, tadalafil prevents the breakdown of cGMP, enhancing and prolonging the erectile response.
2. Benign Prostatic Hyperplasia (BPH):
   • Tadalafil relaxes the smooth muscles in the prostate and bladder, improving urinary flow and reducing symptoms of BPH.
3. Pulmonary Arterial Hypertension (PAH):
   • Tadalafil inhibits PDE5 in the pulmonary vasculature, leading to increased cGMP levels, relaxation of pulmonary smooth muscles, and reduced pulmonary artery pressure.

Tadalafil is approved for the following conditions:

1. Erectile Dysfunction (ED):
   • Treatment of erectile dysfunction in adult men.
2. Benign Prostatic Hyperplasia (BPH):
   • Treatment of lower urinary tract symptoms (LUTS) associated with BPH.
3. Pulmonary Arterial Hypertension (PAH):
   • Treatment of PAH to improve exercise capacity and delay clinical worsening. For PAH, tadalafil is marketed under the brand name Adcirca.
4. Combination of ED and BPH:
   • Tadalafil is also approved for the treatment of men who have both ED and BPH.

The dosage of tadalafil varies depending on the indication and patient response. Below are general guidelines:

1. Erectile Dysfunction:
   • As Needed: 10 mg taken orally at least 30 minutes before sexual activity. The dose may be adjusted to 5 mg or 20 mg based on efficacy and tolerability.
   • Once Daily: 2.5 mg or 5 mg taken orally at the same time each day, regardless of sexual activity.
2. Benign Prostatic Hyperplasia (BPH):
   • Once Daily: 5 mg taken orally at the same time each day.
3. Pulmonary Arterial Hypertension (PAH):
   • Adults: 40 mg taken orally once daily. The dose may be reduced to 20 mg based on tolerability.
   • Children: Dosage is weight-based and should be determined by a healthcare provider.
4. Administration:
   • Tadalafil can be taken with or without food.
   • Avoid excessive alcohol consumption while taking tadalafil, as it may increase the risk of side effects.

Tadalafil is generally well-tolerated, but some side effects may occur. These include:

1. Common Side Effects:
   • Headache
   • Flushing
   • Nasal congestion
   • Dyspepsia (indigestion)
   • Back pain
   • Muscle pain
2. Less Common Side Effects:
   • Dizziness
   • Nausea
   • Visual disturbances (e.g., blurred vision, changes in color perception)
3. Rare but Serious Side Effects:
   • Priapism: Prolonged and painful erection lasting more than 4 hours. This is a medical emergency and requires immediate treatment.
   • Sudden Hearing Loss: Rare cases of hearing impairment have been reported.
   • Cardiovascular Events: Hypotension, myocardial infarction, or stroke, particularly in patients with pre-existing cardiovascular disease.
   • Non-Arteritic Anterior Ischemic Optic Neuropathy (NAION): Sudden vision loss in one or both eyes.

1. Contraindications:
   • Hypersensitivity to tadalafil or any component of the formulation.
   • Concurrent use of nitrates (e.g., nitroglycerin) or nitric oxide donors, as this can cause severe hypotension.
   • Severe cardiovascular disease (e.g., unstable angina, recent myocardial infarction).
2. Warnings:
   • Cardiovascular Risk: Use with caution in patients with a history of cardiovascular disease, as sexual activity may increase cardiac workload.
   • Priapism: Patients should seek immediate medical attention if an erection lasts longer than 4 hours.
   • Vision or Hearing Loss: Discontinue tadalafil and consult a healthcare provider if sudden vision or hearing loss occurs.
3. Monitoring:
   • Monitor for signs of hypotension, especially in patients taking antihypertensive medications.
   • Regular cardiovascular assessment in patients with pre-existing heart conditions.

Tadalafil interacts with several medications, including:

1. Nitrates (e.g., nitroglycerin): Concurrent use can cause severe hypotension and is contraindicated.
2. Alpha-Blockers (e.g., doxazosin): May cause additive hypotensive effects.
3. Protease Inhibitors (e.g., ritonavir): Increase tadalafil levels, requiring dose adjustment.
4. CYP3A4 Inhibitors (e.g., ketoconazole, erythromycin): Increase tadalafil levels, increasing the risk of side effects.
5. Antihypertensive Medications: May cause additive hypotensive effects.

Tadalafil is available under its generic name as well as brand names. Some common formulations include:

• Generic Name: Tadalafil
• Brand Names: Cialis (for erectile dysfunction and BPH), Adcirca (for pulmonary arterial hypertension).