Sildenafil tablets are a prescription medication that relaxes blood vessels to increase blood flow. They’re most commonly used to treat erectile dysfunction (sold as Viagra) and pulmonary arterial hypertension (sold as Revatio). Since the original patent expired, sildenafil has become widely available as a generic, making it one of the most prescribed medications in the world.
How Sildenafil Works
Your body naturally produces a chemical called cGMP that tells smooth muscle in blood vessel walls to relax and widen. An enzyme called PDE5 breaks down cGMP, which keeps this relaxation in check. Sildenafil blocks that enzyme. With PDE5 out of the way, cGMP builds up, blood vessels stay relaxed longer, and blood flow increases to the areas where PDE5 is most active: the penis and the lungs.
This is why sildenafil doesn’t cause an erection on its own. Sexual arousal triggers the release of cGMP in penile tissue. Sildenafil simply prevents that signal from being broken down too quickly, making it easier to get and maintain an erection. Without arousal, there’s no cGMP signal to preserve, and the drug has little noticeable effect in that area.
What Sildenafil Treats
Sildenafil has two distinct FDA-approved uses, each sold under a different brand name and at different doses.
Erectile dysfunction: This is the use most people associate with sildenafil. Under the brand name Viagra, it comes in 25 mg, 50 mg, and 100 mg tablets. The recommended starting dose for most people is 50 mg, taken about an hour before sexual activity. Depending on how well it works and how you tolerate it, the dose can be adjusted down to 25 mg or up to 100 mg.
Pulmonary arterial hypertension (PAH): Under the brand name Revatio, sildenafil treats high blood pressure in the arteries that connect the heart to the lungs. By relaxing those vessel walls, it reduces the strain on the heart. The standard dose for PAH is 20 mg taken three times daily, though the FDA now permits doctors to increase this up to 80 mg three times daily if needed. A lower 5 mg dose was previously approved but has since been revoked after research showed it was less effective.
How Long It Takes to Work
For erectile dysfunction, sildenafil reaches peak levels in your blood within 30 to 120 minutes after taking it on an empty stomach, with a median of about 60 minutes. Most people notice effects starting around the 30-minute mark. The strongest response typically occurs around one to two hours after taking the tablet, with effectiveness gradually declining after that.
The drug can remain active for a surprisingly long time. While the prescribing information notes effects lasting up to four hours (with reduced response compared to the two-hour mark), clinical research has found that sildenafil may remain effective for over 10 hours in some people, with success rates for intercourse dropping only modestly from a peak of about 93% at 1.5 to 2 hours to around 82% beyond the 10-hour mark.
Food, Timing, and Absorption
The official prescribing information notes that eating a high-fat meal before taking sildenafil can reduce how much drug reaches your bloodstream and delay how quickly it gets there. This is why the standard advice is to take it on an empty stomach about an hour before you need it.
That said, real-world research paints a more forgiving picture. In a randomized study comparing sildenafil taken before meals versus with meals, there was no significant difference in how satisfied patients were or how effective the drug was. If you’ve eaten recently, the tablet may take a bit longer to kick in, but it still works. You don’t need to skip dinner.
Common Side Effects
Most side effects are mild and directly related to the blood-vessel-relaxing action of the drug. In a review of 67 placebo-controlled trials, the most frequently reported effects at the 50 mg dose were:
- Headache: 11.4% of patients
- Flushing (warmth or redness in the face): 9.3%
- Indigestion: 2.9%
- Nasal congestion: 2.0%
At the 100 mg dose, the rates were similar for headache (12.2%) and flushing (9.3%), while indigestion increased slightly to 4.8%. These side effects are the same blood-flow changes that make the drug work, just happening in places you didn’t need them. Most people find they fade as the drug wears off.
Some people also notice temporary changes in color vision, typically a blue-green tint, because a related enzyme in the retina gets mildly affected. This is uncommon and resolves on its own.
The Nitrate Interaction
The most important safety concern with sildenafil is its interaction with nitrate medications, which are commonly prescribed for chest pain (angina). Nitrates work by boosting the same cGMP pathway that sildenafil amplifies. Taking both together can cause a dangerous, potentially life-threatening drop in blood pressure. This combination is strictly contraindicated, meaning it should never be used. The same applies to recreational nitrates like amyl nitrite (“poppers”).
Alpha-blockers, used for high blood pressure or prostate enlargement, can also interact with sildenafil and cause low blood pressure. If you take either of these types of medication, your prescriber needs to know before writing a sildenafil prescription.
Brand Name vs. Generic
Viagra and Revatio were both originally made by Pfizer. Since patent protection expired, dozens of generic versions have entered the market. Health Canada’s drug database alone lists numerous approved generic sildenafil products from different manufacturers. In the United States, the generic landscape is similarly broad. Generic sildenafil contains the same active ingredient at the same doses and is held to the same bioequivalence standards, meaning it works the same way. The main difference is price: generic sildenafil costs a fraction of what brand-name Viagra does.
If you’re prescribed sildenafil for erectile dysfunction, you’ll typically receive generic tablets unless you specifically request the brand. For pulmonary arterial hypertension, your doctor may prescribe either the brand or generic depending on your insurance coverage and clinical needs.