Sildenafil information from DrugsUpdate  

Pharmacodynamics

Pharmacokinetics

Sildenafil inhibits phosphodiesterase type-5 (PDE5) which is responsible for cGMP degradation in the corpus cavernosum. Inhibition of PDE5 increases cGMP levels in the corpus cavernosum which results in smooth muscle relaxation and inflow of blood to the corpus cavernosum.

Absorption
Rapidly absorbed after oral admin. Bioavailability: about 40%.

Distribution
Widely distributed. Plasma protein binding: 96%.

Metabolism
Metabolised in the liver mainly by CYP3A4 and CYP2C9.

Excretion
Excreted as metabolites mainly in the faeces.

Sildenafil Indications / Sildenafil Uses

Information Not Available

Sildenafil Adverse Reactions / Sildenafil Side Effects

Headache, flushing, respiratory tract infection, angina pectoris, AV block, migraine, syncope, tachycardia, postural hypotension, MI, cerebral thrombosis, cardiac arrest, paraesthesia, tremor, depression, herpes simplex, skin ulcer, oesophagitis, abnormal LFT, rectal haemorrhage, hypoglycaemic reaction, arthritis, deafness, retinal vascular disease, photosensitivity, accidental fall.

Precautions

Overdosage
Should be managed by standard supportive care.

Special Precautions

Caution when used in patients with anatomical deformation of penis or conditions that may predispose them to priapism (e.g. sickle cell anaemia, myeloma, or leukaemia). Mild, transient, dose-related impairment of colour discrimination (blue/green) may occur. Hepatic or severe renal impairment, bleeding disorders, active peptic ulceration, hypotension, recent history of stroke, MI, arrhythmias, unstable angina, heart failure or retinal disorders. May cause sudden loss or decrease in hearing.

Other Drug Interactions

Inhibitors of CYP3A4 such as cimetidine and erythromycin are likely to reduce sildenafil clearance. CYP3A4 inducers such as rifampicin may decrease the plasma concentrations of sildenafil. Symptomatic hypotension when used with α-blockers. Plasma concentrations are increased by ritonavir.

Potentially Fatal: May potentiate hypotensive effects of of organic nitrates and nicorandil.

Other Interactions

Food Interaction
Concurrent admin with grapefruit juice may lead to increased plasma levels of sildenafil.

Dosage

Oral
Erectile dysfunction
Adult: 50 mg about 1 hour before sexual intercourse. May adjust dose depending on penile response. Max: 100 mg/dose and not to be taken > once in 24 hours.
Elderly: >65 years:   Lower initial dose at 25 mg.
CrCl (ml/min)          Dosage Recommendation
<30                        Initially, 25 mg.
Hepatic impairment: Initially, 25 mg.

Special Populations
For patients taking drugs which are CYP3A4 inhibitors: initially, 25 mg. For patients on ritanavir, not >25 mg every 48 hours. Patients who are stabilised on α-blocker treatment should use an initial dose of 25 mg. Higher doses should not be taken within 4 hours of an α-blocker.

Food(before/after)

May be taken with or without food

Sildenafil and Pregnancy

Either animal-reproduction studies have not demonstrated a foetal risk but there are no controlled studies in pregnant women or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the 1st trimester (and there is no evidence of a risk in later trimesters).

Sildenafil and Lactation

Undetermined

Sildenafil and Children

Safety and efficacy not established

Sildenafil and Geriatic

Revatio
Dose selection should be cautious, usually starting at the lower end of the dosing range, reflecting the greater frequency of decreased hepatic and renal function and comorbidity.

Viagra
Consider initial dose of 25 mg in patients older than 65 years of age.

Sildenafil and Other Contraindications

Hypersensitivity. Patients concurrently or intermittently using organic nitrates in any form.

Storage

Oral
Store at 15-30°C

Lab interference

Oral
Store at 15-30°C