Sildenafil: What It Is, How Viagra Works and What It Is Used For
What is sildenafil? The drug in Viagra, FDA-approved for ED since 1998 and sold as Revatio for lung blood pressure. How it works, side effects, and prices.
By Julian Caraulani · Last updated
TreatmentReviewed by Dr. A. Goher, MBBS
- FDA-approved and compounded
- LegitScript seal ID shown
- 49 states (not AR)
This page covers what sildenafil is, how it works, what it is used for, how long it lasts, food and alcohol, the side effects on its label, and who sells it. Generic sildenafil tablets carry the same FDA labeling as the brand. We do not give doses; the prescriber sets the strength.
What is Viagra?
Viagra is the brand name for sildenafil citrate tablets. The label's first line reads "Initial U.S. Approval: 1998". Generic versions are approved by the FDA as the same active drug, and they are what most online programs sell. Our sildenafil vs Viagra guide explains what the FDA requires of a generic and why the brand costs more.
Chews, mints and sublingual tablets that contain sildenafil are a different thing. A pharmacy mixes them, and they are not FDA-approved products. Each plan in our price tables says which kind it is.
Sildenafil uses: what is sildenafil used for?
MedlinePlus, the US National Library of Medicine's drug guide, lists two uses (MedlinePlus: sildenafil, read 14 November 2026):
- Erectile dysfunction (ED). It says "Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men."
- Lung blood pressure. Sold as Revatio or Liqrev, it treats "pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the lungs, causing shortness of breath, dizziness, and tiredness)".
The ED tablets and the lung tablets are the same drug at different strengths and schedules. Some clinicians prescribe the small Revatio-strength generic for ED. That choice belongs to the prescriber.
MedlinePlus is clear about what sildenafil does not do. Taken for ED, it does not "increase sexual desire". It also "does not prevent pregnancy or the spread of sexually transmitted diseases".
- FDA-approved and compounded
- LegitScript seal ID shown
- 49 states (not AR)
How does Viagra work?
An erection needs blood to flow into the penis and stay there. During arousal, nerves release a chemical messenger called nitric oxide. That raises a second messenger, cGMP, which relaxes the muscle in the penis so blood can flow in. An enzyme called PDE5 breaks cGMP down.
Sildenafil blocks PDE5. The label says it "enhances the effect of NO by inhibiting phosphodiesterase type 5 (PDE5)" (DailyMed: Viagra label). NO is nitric oxide. With PDE5 blocked, cGMP lasts longer and the erection is easier to get and keep.
What does Viagra do, and what does it not do?
It makes an erection easier when you are already aroused. It does not create desire, and it does nothing without arousal. The label says "Sildenafil at recommended doses has no effect in the absence of sexual stimulation." It does not change the cause of ED either; it works for as long as the drug is in your blood.
How long sildenafil lasts
The label says it may be taken "anywhere from 30 minutes to 4 hours before sexual activity". Peak blood levels come at a median of 60 minutes on an empty stomach, and the half-life is about 4 hours. A high-fat meal slows it by about an hour. Our guide to how long Viagra lasts has the full timing.
Sildenafil and alcohol
The label reports a study in healthy men with blood alcohol near the legal driving limit. It found that "sildenafil did not potentiate the hypotensive effect of alcohol in healthy volunteers" (DailyMed: Viagra label). In plain words, it did not make alcohol's drop in blood pressure bigger. Heavy drinking makes erections harder on its own, though.
Medicines that do not mix with sildenafil
The firm rule is nitrates, the chest-pain drugs, including "poppers". The label lists them as a contraindication, which means you must not take the two together, because blood pressure can fall too far. Riociguat, a lung blood pressure drug, is also off limits.
Alpha-blockers, other blood pressure medicines and some antibiotics, antifungals and HIV drugs need care. Our ED pill side effects guide explains each group. Bring your full medicine list to the visit.
Sildenafil side effects
These are from Table 2 of the Viagra label: trials where men took it as needed, against a dummy pill (placebo).
| Side effect | Sildenafil | Placebo | Source |
|---|---|---|---|
| Headache | 16% | 4% | Viagra label, Table 2, read 14 Nov 2026 |
| Flushing | 10% | 1% | Same label |
| Indigestion (dyspepsia) | 7% | 2% | Same label |
| Stuffy nose | 4% | 2% | Same label |
| Abnormal vision | 3% | 0% | Same label |
| Back pain | 2% | 2% | Same label |
| Dizziness | 2% | 1% | Same label |
The vision effect is "predominantly color tinge to vision, but also increased sensitivity to light or blurred vision", per the Viagra label. A pooled study of many trials found sildenafil "had the highest rate of overall adverse events" of the ED pills compared, while also scoring well on how well it worked (Chen et al., Eur Urol 2015, PubMed). Adverse events means side effects.
When to get help
MedlinePlus says to call a doctor right away for "erection that is painful or lasts longer than 4 hours", sudden vision or hearing loss, chest pain or fainting. An erection over 4 hours is an emergency: go to an emergency room.
Who sells generic sildenafil, and what it costs
These programs publish a price that includes the visit. The table is ranked on our score. Prices come from each seller's own page, as checked November 14, 2026.
| Program | Lowest per dose | Cheapest monthly plan |
|---|---|---|
| Ro | $4 | $32 |
| Rex MD | $1.80 | Not published |
| Strut Health | $1.60 | $16 |
| Hims | $4 per use | $22 |
| Idun Rx | $6 | $36 |
| Lemonaid Health | $2 | Consult billed separately |
Every plan, pack and read date is on our sildenafil price page. The per-pill cost guide adds the cash price with your own prescription, and Best online ED programs ranks every program.
Questions
FAQ
What is sildenafil?
Sildenafil is the drug in Viagra. It belongs to a group called PDE5 inhibitors, and the FDA first approved it in 1998 to treat erectile dysfunction. Sold as Revatio, the same drug treats high blood pressure in the lungs. Generic sildenafil tablets are FDA-approved and carry the same label as the brand.
What is Viagra?
Viagra is the brand name for sildenafil citrate tablets, made today by Viatris. Generic sildenafil is the same active drug at a much lower price. Compounded chews and mints that contain sildenafil are made by pharmacies and are not FDA-approved products.
How does Viagra work?
During arousal, the body makes nitric oxide, which raises a messenger called cGMP that lets blood flow into the penis. An enzyme, PDE5, breaks cGMP down. Sildenafil blocks PDE5, so cGMP lasts longer and an erection is easier to get and keep. Without arousal, the label says it has no effect.
What is sildenafil used for?
Two things. For men, it treats erectile dysfunction, sold as Viagra and generics. For adults and children with pulmonary arterial hypertension, which is high blood pressure in the lung arteries, it is sold as Revatio or Liqrev. The two uses rely on the same drug at different strengths and schedules.
Does Viagra increase sex drive?
No. MedlinePlus says sildenafil taken for ED does not increase sexual desire, and the label says it does nothing without sexual stimulation. Men with low desire may have a different cause, such as low testosterone, stress or another medicine. Our guide on whether Viagra increases libido covers those.
Is sildenafil safe with alcohol?
The Viagra label reports that in healthy men with blood alcohol near the driving limit, sildenafil did not make alcohol's drop in blood pressure bigger. Heavy drinking still weakens erections on its own. Tell your prescriber how much you drink so they can judge what is safe for you.
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