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How PDE5 Inhibitors Work in Erectile Dysfunction
The Molecular Mechanism of Sildenafil
Sildenafil citrate, the active ingredient in both Viagra and generic sildenafil, works by selectively inhibiting phosphodiesterase type 5 (PDE5), an enzyme found predominantly in penile blood vessels. When sexual arousal occurs, nitric oxide is released in the penis, which activates the enzyme guanylate cyclase. This increases levels of cyclic guanosine monophosphate (cGMP), a chemical messenger that relaxes smooth muscle cells in blood vessel walls, allowing increased blood flow to create an erection.
PDE5 normally breaks down cGMP, ending the erection. By blocking this enzyme, sildenafil allows cGMP levels to remain elevated for longer periods, maintaining the physiological processes necessary for a sustained erection. This mechanism explains why sexual stimulation is still required – sildenafil enhances the body's natural response rather than creating an automatic reaction.
Pharmacokinetics: Absorption and Duration
After oral administration, sildenafil is rapidly absorbed from the gastrointestinal tract, reaching peak plasma concentrations within 30-120 minutes on an empty stomach. High-fat meals can delay absorption by up to 60 minutes and reduce peak concentration by approximately 29%. The medication has a half-life of approximately 4 hours, meaning effects typically last 4-6 hours, though this doesn't mean an erection persists for this duration.
The liver primarily metabolises sildenafil via the CYP3A4 enzyme system, which is why certain medications that affect this pathway can interact with sildenafil. Understanding these interactions is crucial for safe prescribing, particularly with medications containing nitrates, which can cause dangerous drops in blood pressure when combined with PDE5 inhibitors.
Dosage Optimisation and Clinical Considerations
Both Viagra and generic sildenafil are available in 25mg, 50mg, and 100mg tablets. The typical starting dose is 50mg, taken approximately one hour before sexual activity. Based on individual response and tolerability, the dose may be adjusted to 25mg for those experiencing side effects or increased to 100mg for enhanced effectiveness. The maximum recommended frequency is once per 24-hour period.
Clinical studies have shown that sildenafil is effective in approximately 70-85% of men with erectile dysfunction across various underlying causes, including diabetes, cardiovascular disease, and psychological factors. The medication's selectivity for PDE5 over other phosphodiesterase enzymes contributes to its favourable safety profile when used appropriately.
Safety Profile and Contraindications
Sildenafil has been extensively studied since its introduction in 1998, with a well-established safety profile when used under medical supervision. Common side effects include headache, facial flushing, indigestion, and nasal congestion, which typically resolve as the medication is metabolised. More serious but rare side effects include sudden vision or hearing loss, which require immediate medical attention.
Absolute contraindications include concurrent use of nitrate medications, recent cardiovascular events, severe liver impairment, and certain inherited eye conditions. EverydayMeds ensures thorough clinical assessment by UK-licensed prescribers to identify any contraindications before prescribing either branded Viagra or generic sildenafil, prioritising patient safety whilst providing access to effective treatment options.
