Pharmacology of Drug Sildenafil Citrate

Mechanism of Action
Sildenafil is a phosphodiesterase enzyme (PDE5) inhibitor. This enzyme is responsible for the degradation cyclic guanosine monophosphate (cGMP) produced in reaction to sexual stimulation. The more cGMP is available, the more durable the erection. Sildenafil inhibits the PDE5 enzyme, preserving cGMP levels, therefore aiding erection viability and durability Erection of the penis is caused by the filling of the penis with blood. Filling occurs because the blood vessels that bring blood to the penis increase in size and deliver more blood to the penis and at the same time, the blood vessels that take blood away from the penis decrease in size and remove less blood from the penis. Sexual stimulation that leads to an erection causes the production and release of nitric oxide in the corpora cavernosa of the penis.

The nitric oxide causes an enzyme guanylate cyclase, to produce cyclic guanosine monophosphate (cGMP). It is the cGMP that is primarily responsible for increasing and decreasing the size of the blood vessels carrying blood to and from the penis, respectively, and causing the erection. When the cGMP is destroyed by enzyme phosphodiesterase-5, the blood vessels return to their normal size, blood leaves the penis, and the erection ends. Sildenafil prevents phosphodiesterase-5 from destroying cGMP so that cGMP stays around longer. The persistence of cGMP leads to a more prolonged engorgement of the penis with blood. It relaxes muscles and increases blood flow to particular areas of the body.

Pharmaco Kinetics

Absorption: It is readily absorbed from the bloodstream and the bioavailability is about 40%. Distribution: It is widely distributed in protein bound form (96% bound to plasma protein) mostly to tissues. Metabolism: It is metabolized in the liver by an enzyme known as CYP3A4 and others to produce active N-Desmethyl derivatives. Excretion: It is primarily excreted in the feces (80%) and small amount in the urine

Onset Of Action
Within 30 minutes

Half Life
4 hours

Duration Of Action
4 hours

Adverse Effects

1. Facial flushing 2. Headaches 3. Stomach upset 4. Diarrhea 5. Flu-like symptoms 6. Nausea 7. Back pain 8. Muscle aches 9. Low blood pressure 10. Blurred vision 11. Abnormal ejaculation 12. Prolonged erections 13. Sweating 14. General ill feeling 15. Irregular heartbeat 16. Swelling in hands, ankles, or feet 17. Shortness of breath 18. Lightheadedness 19. Fainting In pre-existing heart disease: 1. Chest pain 2. Heart attacks 3. Death 4. Strokes 5. Palpitations and increased heart rate 6. Vision changes

1. Hypersensitivity to Sildenafil 2. Patients on nitrate therapy 3. Patients taking a adrenergic receptor antagonist

Special Precautions

1. Do not take Sildenafil more than once a day 2. Contact doctor if erection is painful or lasts longer than 4 hours 3. Heart disease 4. Recent history a heart attack (within the past 90 days) 5. Recent history of stroke or congestive heart failure (within the past 6 months) 6. Angina 7. Hypotension 8. Hypertension 9. Liver disease 10. Kidney disease 11. Sickle cell anemia 12. Multiple myeloma 13. Leukemia 14. Haemophilia 15. Stomach ulcer 16. Retinitis pigmentosa 17. Peyronie`s disease (physical deformity of the penis) 18. Diabetes 19. High cholesterol 20. Pre-existing eye problems 21. One who smoke or are over 50 years old



Breast Feeding

Old Age

Use with caution





Non-specific beta-blockers, loop and potassium-sparing diuretics enhance efficacy of sildenafil. Concomitant administration of sildenafil 100mg and amlodipine 5 or 10mg in hypertensive patients led to a mean additional blood pressure reduction of 8mm Hg systolic and 7mm Hg diastolic. Cytochrome P450 inhibitors like erythromycin, ketoconazole, itraconazole are likely to reduce sildenafil clearance.


1. Erectile dysfunction


Adult: 50 mg taken, 1 hour before sexual activity or anywhere from 0.5 – 4 hours before sexual activity. Based on effectiveness and toleration, the dose may be increased to a maximum 100 mg or decreased to 25 mg. The maximum recommended dosing frequency is once per day.



Store at room temperature between 15-30°C. Protect from moisture and heat. Keep out of the reach of children.

Over Dose

No overdose, if any overdose occurs then seeks immediate medical attention. Give supportive measures and symptomatic treatment.