Before using Viagra (Sildenafil)

Viagra (Sildenafil) is a medication available with a doctor’s prescription. Nowadays anyone can purchase Viagra online. Whether you buy Viagra in a local pharmacy with a prescription or online without prescription it is recommended to use this medication only after a consultation with a doctor. Viagra contradict with certain medical health conditions you may have or with certain drugs you may take. So it is absolutely necessary to visit your doctor to find the right dose for you and if you have other unrelated conditions – undergo special tests.

Viagra is contraindicated with any nitrate-based medications taken for chest pain or heart disease. Viagra is not allowed to be used with other blood pressure lowering medications. Viagra in combination with any blood lowering medication may provoke a serious blood pressure reduction and lead to dangerous health state.

Other unrelated conditions that contradict with Viagra intake include:

  • a recent history of a heart attack, stroke, or congestive heart failure;
  • heart disease or heart rhythm problems;
  • high or low blood pressure;
  • coronary artery disease;
  • liver or kidney impairment;
  • sickle cell anemia, multiple myeloma, leukemia, hemophilia;
  • a stomach ulcer;
  • retinitis pigmentosa;
  • a history of priapism condition (a prolonged and painful penile erection);
  • Peyronie’s disease;
  • traumas to the genital area.

If you have the mentioned above conditions you may need special tests before using Viagra. Your doctor will adjust you the necessary dose which you should not change without talking to your doctor first.

If your doctor recommends you to avoid sexual activity due to your serious cardiovascular status, do not take Viagra.


Viagra metabolism and elimination

Viagra (Sildenafil) is refined basically by the CYP3A4 (main way) and CYP2C9 (secondary way) hepatic microsomal isoenzymes. The basic Sildenafil metabolite is a result from N-desmethylation, and undergoes further metabolism. This main metabolite restricts selectively PDEs like Sildenafil and shows the properties for PDE5 about 50% of the parent medication. This metabolite has 40% plasma levels of those noted for Sildenafil, and thus the metabolite amounts to 20% of Viagra’s pharmacologic effects.

Sildenafil is eliminated as metabolites basically in the feces – about 80% of the received dose and in the urine – about 13% of the dose.

The half life of Sildenafil is about 4 hours, this means that during a period of 4 hours a person may get and maintain an erection. Still the response of a patient decreases after 2 hours after the oral dose. So it is recommended to take Viagra 30 minutes – 2 hours before sexual activity to receive the most pleasant results.


Viagra effect on blood pressure

Viagra effect on blood pressure was estimated during special clinical trials which revealed  that the max 100 mg dose of Sildenafil given  to healthy test participants decreases  sitting blood pressure. The reduction in systolic/diastolic blood pressure noted in the participants was maximum 8.3/5.3 mmHg. Such blood pressure decrease was mostly observed about 1–2 hours after oral dose.

The same Viagra effect on blood pressure was observed with other doses 25 mg, 50 mg. So the effect on blood pressure is not associated to dose or plasma levels within this dosage range.

The serious Viagra effect on blood pressure was noted in patients receiving nitrate- containing medications. The concomitant use of Viagra with nitrates is contraindicated because it can produce dangerous drop in blood pressure. Men with ED and uncontrolled blood pressure or taking nitrates should consult a doctor before using Viagra.

Effects of Viagra on erectile response

Eight double-blind, placebo-controlled crossover studies were conducted to evaluate the effects of Viagra on erectile response in patients having problems with getting and keeping erections.

The patients taking part in the studies had either organic or psychogenic erectile dysfunction. The administration of Viagra to the participants (with the sexual stimulation) led to improved erections. It was estimated with the help of an objective measurement of hardness and duration of erections (RigiScan).

Most studies proved the efficacy of Viagra about an hour after the dose adjustment. The erectile response significantly increased with the dose enlargement and plasma concentration. The duration of the effect of Viagra was evaluated and stated up to 4 hours.

Viagra Mechanism of Action

The physiologically penile erection requires activation of several processes such as local release of nitric oxide (NO) in the corpus cavernosum when sexual stimulation occurs, the production of enzyme guanylate cyclase  and production of  cyclic guanosine monophosphate (cGMP. All these processes result in smooth muscle relaxation in the corpus cavernosum and extra inflow of blood into penile tissues.

Sildenafil enhances the effect of nitric oxide by restricting phosphodiesterase type 5 (PDE5). PDE5 is known for its effect to diminish cGMP in the corpus cavernosum. When sexual stimulation occurs nitric oxide is released, Sildenafil inhibits PDE5 and this causes enlarged levels of cGMP in the corpus cavernosum. The whole process has a smooth muscle relaxation effect. Sildenafil requires sexual stimulation to start working.