• Sildenafil – How it works


    Sildenafil is a prescription drug used for the treatment of ED. It is also used to treat pulmonary arterial hypertension, which is characterized by consistently high blood pressure in the pulmonary artery (the artery that delivers blood from the right ventricle of the heart to the lungs).

    How Sildenafil is Taken

    Sildenafil is available in 20-mg, 25-mg, 50-mg, and 100-mg tablets. The typical dose is 50 mg taken one hour before sexual activity. Sildenafil may, however, be taken anywhere from a half an hour to four hours before sexual activity. The dose may be increased to 100 mg or decreased to 25 mg depending on effectiveness and side effects. For the treatment of ED, sildenafil is not to be taken more than once in a 24-hour period.

    For the treatment of pulmonary arterial hypertension, sildenafil is given at a dosage of 20 mg three times a day.

    How Sildenafil Works

    Male erections involve the release of nitric oxide in the corpus cavernosum of the penis during sexual stimulation. Nitric oxide is a gas that has several physiological functions, including relaxation of blood vessels. It stimulates the production of cyclic guanosine monophosphate (cGMP). In the corpus cavernosum, cGMP relaxes blood vessels and facilitates blood flow into the penis. This inflow of blood produces an erection.

    Sildenafil, like other drugs in its class, inhibits the enzyme phosphodiesterase type 5 (PDE-5). This enzyme breaks down cGMP. Inhibition of this enzyme causes cGMP to accumulate and increase blood flow into the penis.

    Sildenafil is unable to initiate an erection by itself. It helps to strengthen and maintain an erection that was initiated by sexual stimulation.

    Sildenafil relieves pulmonary arterial hypertension in the same way that it facilitates penile erection. In the pulmonary artery, sildenafil causes cGMP to accumulate and this promotes relaxation of the blood vessel and the reduction of blood pressure.

    How the Body Affects Sildenafil

    Sildenafil concentrations reach a maximum in the blood between 30 and 120 minutes after administration. It is metabolized, or broken down, in the liver by enzymes (particularly the CYP3A4 enzyme). The compound that results from this metabolism is as active as sildenafil itself. The amount of time needed for the blood concentrations of sildenafil and this active metabolite to be reduced by half, the half-life, is approximately four hours. The majority of the sildenafil administered is excreted in the feces.

    Risks and Precautions

    Some people are susceptible to dangerously low blood pressure when at rest and lying down because of sildenafil’s ability to enlarge blood vessels:

    • those with slow or limited blood flow out of the heart
    • those whose blood pressure is not controlled properly by the body

    Sildenafil may produce prolonged erections (lasting longer than four hours) or painful erections that last more than six hours in duration (praipism). The risk of priapism is greater in people who are already at high risk, such as those with sickle cell anemia or some cancers.

    Sildenafil is used with caution in people with abnormal penis structure (e.g., angulation or Peyronie disease ).

    Interesting Facts

    • Sildenafil is one of the most studied drugs.
    • Sildenafil is a very popular recreational drug, especially in teenagers