Viagra and Its Active Ingredient — A Clinical Overview

Medication Overview

What Is Sildenafil and How Does Viagra Work?

Sildenafil citrate is the active ingredient in Viagra, an oral tablet originally developed by Pfizer researchers in the late 1980s as a treatment for hypertension and angina. When clinical trials unexpectedly revealed its effect on erectile function, Pfizer redirected development toward erectile dysfunction (ED). On March 27, 1998, the FDA granted approval — making sildenafil the first oral drug confirmed safe and effective for ED in the United States. It is also approved under the brand name Revatio for pulmonary arterial hypertension.

The Mechanism: Blood Flow and PDE5 Inhibition

An erection depends on a precisely timed sequence of chemical signals. Sexual stimulation causes the nervous system to release nitric oxide into the smooth muscle tissue of the penis. Nitric oxide activates an enzyme called guanylate cyclase, which produces cyclic guanosine monophosphate (cGMP). This compound relaxes arterial smooth muscle and allows blood to fill the erectile chambers of the penis.

The problem in erectile dysfunction is that phosphodiesterase type 5 (PDE5), an enzyme present in penile tissue, degrades cGMP before an erection can fully develop or be maintained. Sildenafil selectively blocks PDE5, allowing cGMP to persist at effective concentrations. The result is sustained arterial dilation and the ability to achieve an erection during sexual activity. Sildenafil has no effect in the absence of arousal — it amplifies the natural response rather than triggering one independently.

Who Is Sildenafil Prescribed For?

Sildenafil is indicated for adult men whose erectile dysfunction stems from organic, psychological, or mixed causes. The most common underlying conditions that bring men to a prescription include cardiovascular disease, type 2 diabetes, hypertension, and performance anxiety. Clinical trials confirm that sildenafil improves erections in approximately 69 to 82% of men with ED, making it one of the most effective first-line treatments in modern urology.

Men with diabetes tend to need higher doses and may see somewhat lower response rates — around 55 to 65% — due to the combined vascular and neuropathic damage that diabetes causes to penile tissue. Men with purely psychological ED generally respond very well, even at lower doses.

Sildenafil is also prescribed for pulmonary arterial hypertension (PAH), where it reduces pulmonary vascular resistance and improves exercise capacity. In this context it is dosed three times daily at 20 mg rather than as a single pre-activity dose.

Standard Doses: 25 mg, 50 mg, and 100 mg

Sildenafil for erectile dysfunction is available in three oral tablet strengths. Most prescribing guidelines recommend starting at 50 mg and adjusting based on the patient's response and tolerability:

  • 25 mg — the conservative starting point for men over 65, those with mild liver or kidney impairment, or those taking medications that slow sildenafil metabolism via the CYP3A4 enzyme pathway.
  • 50 mg — the standard starting dose for most adult men; produces successful erections in a majority of patients who do not have severe underlying vascular damage.
  • 100 mg — the maximum single dose, recommended when 50 mg does not produce an adequate response. Survey data from large clinical trials showed 82% of men at this dose reported improved erections, versus 74% at 50 mg. Side effects are more pronounced at this strength.

Timing and Food Interactions

Sildenafil reaches peak plasma concentration approximately 30 to 60 minutes after ingestion and remains pharmacologically active for roughly four to six hours. High-fat meals delay absorption and reduce peak plasma levels — a relevant practical point for men who take the medication after a large dinner. A light meal or an empty stomach produces the most predictable onset. Alcohol consumed in parallel can amplify the blood-pressure-lowering effect and impair the natural arousal response needed for the drug to work.

Safety and Context

When Sildenafil May Not Be Suitable

Sildenafil is contraindicated in several situations that physicians assess before prescribing:

  • Nitrate medications: men taking any organic nitrate for chest pain — including nitroglycerin patches or tablets, isosorbide mononitrate, or isosorbide dinitrate — must not take sildenafil. The combination causes an additive blood-pressure drop that can be severe and life-threatening. This contraindication is absolute.
  • Recent cardiovascular event: men who have had a heart attack or stroke within the past 90 days, or who have unstable angina, should not use sildenafil until cleared by a cardiologist.
  • Severe liver disease: the liver is the primary site of sildenafil metabolism; severe hepatic impairment significantly raises drug levels and adverse event risk.
  • Certain eye conditions: men with a history of non-arteritic anterior ischemic optic neuropathy (NAION) in one eye are advised to avoid PDE5 inhibitors.

Common Side Effects

Sildenafil has a well-characterized safety profile after more than two decades of clinical use. The most frequently reported effects are dose-dependent and generally mild:

  • Headache (up to 16% of users at 100 mg)
  • Facial flushing — warmth and redness in the face and neck
  • Nasal congestion
  • Visual disturbances — a transient blue-green tinge to vision in a small percentage of users, caused by mild inhibition of PDE6 in the retinal photoreceptors
  • Indigestion
  • Dizziness or mild drop in blood pressure when standing quickly

These effects typically resolve as the drug clears the system. Men who find 100 mg too uncomfortable often find 50 mg provides an acceptable balance of efficacy and tolerability.

Generic Sildenafil Since 2017

Pfizer's U.S. patent on Viagra expired in December 2017, and multiple manufacturers — including Teva and Greenstone — now produce generic sildenafil tablets that have passed FDA bioequivalence requirements. The active molecule and dose are identical to the branded product. The practical difference is dramatic: brand-name Viagra can cost $60–85 per tablet at a retail pharmacy without insurance, while generic sildenafil from the same pharmacy often costs under $2.00 per tablet. For men who pay out of pocket, choosing a generic from a licensed pharmacy is the straightforward, clinically equivalent option.

About This Page

This article is intended for informational purposes and does not constitute medical advice or a treatment recommendation. Clinical data and dosing figures are sourced from FDA prescribing information and peer-reviewed pharmacology literature. Reviewed by a medical writing team, April 2026.