Clinical Pharmacology and Mechanism of Action
Phosphodiesterase Type 5 Inhibition
According to BNF guidance, sildenafil works by selectively inhibiting phosphodiesterase type 5 (PDE5), an enzyme found in high concentrations within penile erectile tissue. During sexual arousal, nitric oxide is released, which activates guanylate cyclase and increases cyclic guanosine monophosphate (cGMP) levels. This process promotes smooth muscle relaxation and increased blood flow to the corpus cavernosum. PDE5 normally breaks down cGMP, but sildenafil's inhibition allows sustained vasodilation and improved erectile function.
Pharmacokinetic Profile
The BNF details sildenafil's absorption characteristics, with peak plasma concentrations typically achieved within 30-120 minutes after oral administration. Bioavailability is approximately 40% due to first-pass hepatic metabolism via cytochrome P450 3A4 (CYP3A4) pathways. Food intake, particularly high-fat meals, may delay absorption by up to 60 minutes. The elimination half-life ranges from 3-5 hours in most patients, though this may be prolonged in elderly individuals or those with hepatic impairment.
Dosage and Clinical Effectiveness
BNF recommendations suggest initiating treatment with 50mg sildenafil, taken approximately one hour before sexual activity. Clinical studies demonstrate that this starting dose provides adequate response in 60-70% of men with erectile dysfunction. Dose adjustments to 25mg or 100mg may be considered based on individual efficacy and tolerability. The maximum recommended frequency is once daily, with some patients requiring dose reduction if experiencing adverse effects.
Cardiovascular Considerations
The BNF emphasises important cardiovascular safety considerations for sildenafil use. The medication causes mild systemic vasodilation, typically resulting in small decreases in blood pressure (8-10 mmHg systolic, 5-6 mmHg diastolic). This effect is generally well-tolerated in healthy individuals but requires careful assessment in patients with underlying cardiovascular conditions. Contraindications include concurrent nitrate therapy due to potentially dangerous hypotensive interactions.
Drug Interactions and Contraindications
BNF guidance highlights critical drug interactions, particularly with nitrates and riociguat, where concurrent use is absolutely contraindicated due to severe hypotension risk. CYP3A4 inhibitors such as ketoconazole, ritonavir, and erythromycin may increase sildenafil plasma concentrations, necessitating dose reduction. Alpha-blockers may enhance hypotensive effects, requiring careful monitoring when used together.
Treatment Options at EverydayMeds
Following appropriate clinical consultation, patients can choose between generic sildenafil and branded Viagra, both containing the same active ingredient and demonstrating equivalent therapeutic effects. Generic sildenafil typically offers cost savings whilst maintaining the same safety and efficacy profile outlined in BNF guidance. Our UK-licensed prescribers conduct thorough assessments to ensure treatment suitability and optimal dosing recommendations based on individual patient factors.
