What is Viagra Sidenafil?

Background

Viagra (generic name Sildenafil) was the first of the PDE5 inhibitor drugs to be introduced.

Viagra was initially invented as a pill to try to treat angina in patients with ischemic heart disease. Viagra didn’t work as well as the existing nitrates for dilation of blood vessels in the heart, but was noted to have an unusual side effect: significantly improved erections. The side effect was noted with enthusiasm by the manufacturer, Pfizer, and clinical trials were initiated using Viagra to treat erectile dysfunction.

Viagra was approved by the FDA in 1998. It initially was marketed under the slogan “There’s no other tablet proven to work better”, but upcoming comparator trials between Viagra, Levitra and Cialis may prove otherwise.

Since Viagra was the first PDE5 inhibitor medication to be introduced, it has been well-scrutinized. Clinical trials worldwide have established its relative safety and efficacy across a broad spectrum of patients [Ref].

Followup studies have shown the benefits of Viagra are long-term. Among patients on Viagra therapy for 1-3 years, 95% reported that they were satisfied with their treatment of erectile dysfunction over the full 3 years of the study.

Viagra Dosage and usage

Viagra is packaged as blue, diamond-shaped tablets available in three different doses: 25mg, 50mg and 100mg.

The starting dose is normally 50mg. For patients who are older than 65 years, or with significant hepatic impairment or with significant renal impairment, a starting dose of 25mg can be considered. The starting dose is often inadequate, and many men will have to titrate to higher doses to obtain optimal results.

Viagra is taken on an as-needed basis, approximately 1 hour before anticipated sexual activity. The median time of maximum plasma concentration is 60 minutes, though it may be achieved in as quick as 30 minutes or as long as 120 minutes. Patients with impaired absorption or severe erectile dysfunction may have optimal results if they wait the full hour after taking the Viagra.

Meals, especially high-fat meals, significantly decrease the absorption of Viagra, so it is best taken on an empty stomach. Fasting for 2 hours before taking Viagra will provide the most potent response and avoid delayed onset.

Viagra requires sexual stimulation to induce erections. It has a half-life of 3 to 5 hours.

Guidelines recommend as many as eight attempts at using the pill before abandoning and considering other options. However, some urologists note that within their practices, patients often won’t choose to take therapy that long without response.

Viagra Side-effects

Tolerability of Viagra is satisfactory. Commonest reported side-effects include headache (15.8%), flushing (10.5%), dyspepsia (6.5%), nasal congestion (4.2%), respiratory tract infection (4.2%), flu syndrome (3.3%), urinary tract infection (3.1%), abnormal blue/green vision or blurred vision (2.7%). Other rarer side-effects include diarrhea, dizziness, rash, back pain and arthralgia [Ref]. There are case reports of pripasm [Ref].

The visual effects are due Viagra’s effect on a similar enzyme in the eye. There is an enzyme called PDE6 in the retina, and while Viagra is 10-fold more specific for PDE5 in the penis than PDE6 in the eye, there is still a risk involved of retinal problems [Ref].

Viagra temporarily lowers the blood pressure, lowering systolic blood pressure by 8mm Hg when given to a healthy male [Ref]. However, when a healthy male takes both a sublingual nitrate and Viagra, there is a 32mm Hg drop in blood pressure, which leads to symptoms of hypotension [Ref].

Viagra Contraindications, drug interactions and cautions

Concurrent use of any form of nitrates in an important contraindication. Viagra should not be used by any individual who is taking any form of nitrates, due to the aforementioned drop in blood pressure, which may be life threatening.

Use is also contraindicated with alpha-blocker medications because of concerns of hypotension, priapism or other adverse effects.

Since Viagra is metabolized by certain enzymes in the liver, other drugs that inhibit this enzyme can cause Viagra plasma levels to rise excessively high. These drugs include erythromycin, saquinavir, ritonavir, ketoconazole and itraconazole [Ref].

Viagra is not recommended to be used together with other erectile dysfunction drugs [Ref].

Men with the disease of the eye called retinitis pigmentosa should not take Viagra, because a minority of these cases are due to a PDE disorder of the eye.

For the following groups, caution should be used if Viagra is be considered, since fully controlled clinical trials aren’t available. These groups include patients with either resting blood pressure over 170/110 or under 90/50, or a myocardial infarction, stroke, or a life-threatening arrhythmia within 6 months.

Despite some concerns when Viagra was first launched, it can be safely used in men with stable heart disease as long as there is no other contraindications.

Strengths and weaknesses

Viagra was the first PDE5 inhibitor drug to market, so it has the advantage of the most studies behind it. While most of the data learned about side-effects, efficacy and interaction can also apply to the other PDE5 drugs, there is some subtle differences.

Viagra’s onset of action (1 hour) is slower than Levitra’s. Viagra has a slower duration of effect compared to Cialis.

Viagra has a lessened clinical effect when taken soon after a high-fat meal.

One issue that obtains heavy media coverage is the association of Viagra with an eye disease called nonarteritic anterior ischemic optic neuropathy (NAION). NAION causes a loss of vision or even blindness when a blood vessel in the eye is blocked. There was a spike in media coverage following a 2005 paper which detailed seven more case reports of NAION after ingesting Viagra. However, the case reports were in patients with diseases including diabetes, and these diseases already place them at high risk for that eye disease. Also, on a pure numbers scale, there is over 20 million men who have taken Viagra, and the FDA has thus far only received about 50 case reports about the drug provoking NAION. Cialis has had fewer case reports of NAION, and Levitra has had none, but one must bear in mind that Cialis and Levitra also have not been used as often as Viagra.

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