Mechanism of Action

Viagra, known generically as sildenafil, is a phosphodiesterase type 5 (PDE-5) inhibitor. It enhances erection by increasing blood flow to the penis. This is facilitated through the relaxation of smooth muscle of the corpus cavernosum, which occurs through the inhibition of PDE-5. As a result, cyclic guanosine monophosphate (cGMP) levels are maintained, leading to prolonged vasodilation and engorgement of erectile tissues.

Indications

Viagra is primarily indicated for the treatment of erectile dysfunction (ED) in adult men. It is effective in a variety of etiologies including psychogenic, organic, and mixed causes of erectile dysfunction. It has also shown efficacy in the treatment of pulmonary arterial hypertension (PAH) due to its vasodilatory effects on the pulmonary vasculature.

Dosage and Administration

Viagra is typically initiated at a dose of 50 mg to be taken as needed, approximately one hour before sexual activity. Depending on efficacy and tolerability, the dose may be increased to a maximum recommended dose of 100 mg or decreased to 25 mg. It is advised to take Viagra no more than once daily. The medication can be ingested with or without food, although a high-fat meal may delay its onset of action.

Pharmacokinetics

After oral administration, Viagra is rapidly absorbed with peak plasma concentrations occurring within 30 to 120 minutes. The drug has an absolute bioavailability of approximately 40%. It is extensively metabolized in the liver primarily by the cytochrome P450 3A4 (CYP3A4) enzyme system. The elimination half-life of sildenafil is about 4 hours, facilitating a relatively short duration of action.

Contraindications

Viagra is contraindicated in patients using nitrates, either regularly or intermittently, due to the risk of a severe drop in blood pressure. It should not be used in patients with known hypersensitivity to sildenafil or any component of the tablet. Additionally, it is contraindicated in those who are advised against sexual activity due to cardiovascular risk factors post-myocardial infarction or stroke.

Drug Interactions

Concurrent use of Viagra with potent CYP3A4 inhibitors such as ritonavir or ketoconazole can significantly increase sildenafil plasma levels. Caution is warranted when co-administering Viagra with alpha-blockers, as this combination can result in symptomatic hypotension. The efficacy of sildenafil may be reduced when taken with large amounts of alcohol.

Side Effects

Common adverse effects include headache, flushing, dyspepsia, and nasal congestion. Visual disturbances such as changes in color vision or blurred vision occur less frequently but are noted. Serious adverse effects, albeit rare, include priapism and sudden hearing loss. If any severe reactions occur, discontinuation and medical consultation are recommended.

Special Populations

In elderly patients, particularly those over 65, the clearance of sildenafil may be reduced, necessitating a lower starting dose of 25 mg. For patients with severe renal impairment (creatinine clearance <30 mL/min), a lower starting dose is also advised. Hepatic impairment can affect the metabolism of Viagra, and caution or dose adjustment may be necessary.

Monitoring

Regular follow-up assessments are advised to gauge therapeutic efficacy and tolerance. Blood pressure monitoring is crucial, particularly in patients with predisposing cardiovascular conditions. Periodic evaluation should also consider the potential impact on vision and hearing, given the possibility of adverse ocular and auditory effects.

Storage

Viagra should be stored at room temperature, away from moisture and heat, to maintain its stability. Keeping the medication out of reach of children and in its original container can prevent accidental ingestion or misuse. Proper storage ensures the medication’s efficacy and safety until the expiration date.

Patient Education

Patients should be counseled to take Viagra as prescribed and to notify their healthcare provider if they experience any adverse effects. They should be informed about the potential interaction with nitrates and advised to undergo regular follow-up. Educating patients about the importance of adherence to the prescribed dosing regimen can help maximize the therapeutic benefits while minimizing risks.