The Clinical Pharmacology of Viagra

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In order to understand the nature of Viagra, including its pharmacological properties and its effect on patients with E.D. it is first important to outline how the erection process actually works and then detail how Viagra interacts with that erection process.

The Erection Process

An erection is basically a hydraulic event. Blood flows into the penis and when enough pressure builds up, the penis expands and becomes stiff. Sounds simple enough, right? Not really. Getting to that point takes an amazing amount of coordination and interaction between arteries, veins, tissue, hormones, blood vessels, nerves and obviously the human brain. So how does that process actually work?

The erection process begins after any of the five senses interpret something to be sexually stimulating. If arousal occurs due to a visual experience, or after hearing something sexy, or after smelling something that is interpreted to be sexy, this creates an electrical impulse in the brain that travels down the spinal cord jumping from nerve to nerve. When the signal reaches the pelvis area the penile arteries are asked to open up in order to let blood enter to cause an erection.

If stimulations is physical (e.g. direct stimulation to the penis instead of visual stimulation) the nerve pathway that comes into play is a bit different. In this case the sexual message starts out in the pelvic region and is carried by the pudendal nerve (the nerve that innervates the external genitalia). This pudendal nerve connects to what are known as sacral nerves in the lower spine. Those sacral nerves are then responsible for signaling to the penile arteries that they need to relax in order to let blood enter and cause an erection.

The Role of Nitric Oxide in the Erection Process

Nitric Oxide (NO) is a chemical messenger that nerves use in order to communicate with one another. When they come into play in the male groin area they help release other important chemicals or compounds such as cGMP (cyclic guanosine monophosphate) which is directly responsible for vascular smooth muscle relaxation in the penis. The more Nitric Oxide that is present the more cCMP that will be released and the easier it will be for the nerves to signal to the penis that it needs relax and become erect.

cGMP vs. PDE5

cGMP helps cause an erection. PDE5 stops that erection.

cGMP makes the smooth muscles and arteries in the penis relax in order to cause the erection.

PDE5 (phosphodiesterase type 5) is an enzyme that breaks down cyclic guanosine monophosphate (cGMP) during the erection until it finally causes the penis to return to its naturally flaccid state. Without PDE5 the erection would not stop at all and this is Mother Nature’s way of helping us calm down after sexual activity. Unfortunately PDE5 adversely affects men with Erectile Dysfunction.

How Viagra Helps the Erection Process

Viagra is a PDE5 inhibitor which means it helps prevent PDE5 from making our penis flaccid. By putting a brake on PDE5 Viagra allows cGMP to get to work. Greater amounts of cGMP ultimately result in a longer, stiffer and more desirable erection. By inhibiting PDE5 Viagra also enhances the effect of Nitric Acid and nerves can more easily communicate with the sexual area.

Viagra: Pharmacodynamics / Pharmacokinetics

Generic Name: Sildenafil Citrate

Available dosages: 25 mg, 50 mg, 100 mg. The normal starting dosage is usually 50 mg but your doctor should be the one to determine how much you need.

Absorption: Sildenafil Citrate will normally be absorbed into the blood stream and will start to work from 30 minutes to 120 minutes after ingestion. Though some men have experienced erections in as little as 14 minutes. Note: Foods that are high in saturated fat and cholesterol may delay the absorption time.

Distribution: Sildenafil Citrate is distributed to the tissues and binds itself to proteins. Very little is passed on to the semen. (less than 0.001%) So it will not likely have any effect on reproduction.

Half life: approximately 4 hours. Men can possibly have several sexual encounters while Viagra remains in the blood stream.

Excretion / Elimination: Approximately 80 % of Viagra leaves the body through the feces the rest is eliminated through the urine (13 %) and other glands.

Effects on Blood Pressure: Viagra is a PDE5 inhibitor. All PDE5 inhibitors tend to lower your blood pressure. The greatest drop in blood pressure will occur approximately 1 to 2 hours after ingestion. According to Pfizer there is a “mean maximum decrease in systolic/diastolic blood pressure of 8.4/5.5 mmHg”. For this reason, if you are taking low blood pressure medications or nitrates be sure to let your doctor know before you ask for a Sildenafil Citrate prescription. You may be able to temporarily substitute Nitrates for other medications in order to take Viagra but never use them together. As this may cause a dangerous drop in blood pressure and may produce a stroke or even death.

Effects on Vision: Some patients may experience blurred vision or may begin to see a blue/green tinge after taking Viagra.