What sildenafil actually does
An erection is a blood-flow event. Sexual arousal triggers nerve endings in the penis to release nitric oxide, which raises levels of a signalling molecule called cyclic GMP. Cyclic GMP relaxes the smooth muscle inside the two spongy chambers of the penis (the corpora cavernosa), those chambers fill with blood, and the pressure compresses the outflow veins so the blood stays put.
An enzyme called phosphodiesterase type 5 (PDE5) breaks cyclic GMP down again. Sildenafil blocks that enzyme. The practical effect is that the normal arousal signal produces a firmer erection and holds it for longer.
Because PDE5 also exists in other tissues — blood vessels throughout the body, the oesophageal sphincter, the retina — blocking it produces the predictable side-effect profile described below.
How well it works
Across the large randomised trials that supported approval, sildenafil improved erections in roughly 60–70% of men with ED from mixed causes. That number moves considerably depending on why you have ED:
| Group | Typical response | Notes |
|---|---|---|
| Psychogenic / mixed ED | 70–85% | The most responsive group. Often works on the first proper attempt. |
| Long-standing type 2 diabetes | 50–60% | Nerve and small-vessel damage limits the ceiling. Higher doses often needed. |
| After radical prostatectomy | 35–75% | Depends heavily on whether the nerve bundles were spared and how much time has passed. |
| After pelvic radiotherapy | 50–60% | Response can decline over years as radiation effects progress. |
| Severe vascular disease | 40–50% | Often a signal to investigate the arteries elsewhere in the body. |
Guidelines are clear that a medication should not be called a failure until you have had an adequate trial: the right dose, taken correctly, on at least four to eight separate occasions. A meaningful proportion of apparent non-responders succeed after nothing more than instruction on timing, food and the need for stimulation.
Doses and how to take it
| Dose | When it is used |
|---|---|
| 25 mg | Starting dose for men over 65, significant liver or kidney impairment, or when taking interacting medicines. |
| 50 mg | The usual starting dose for most adults. |
| 100 mg | Maximum single dose. Used when 50 mg gives a partial response with tolerable side effects. |
Timing
- Take it 30–60 minutes before you expect to have sex. The useful window extends for several hours afterwards.
- A large or high-fat meal can delay absorption by an hour or more. If you have eaten a steak dinner, take it earlier or wait longer.
- Alcohol in moderation does not block the drug, but alcohol itself impairs erections and adds to the blood-pressure-lowering effect. Heavy drinking undermines the whole exercise.
If your first attempt disappoints
- Confirm the basics: taken on a light stomach, enough time before sex, and with genuine sexual stimulation.
- Repeat the same dose several times before changing anything. Performance anxiety on the first attempt is extremely common and resolves with repetition.
- Ask your prescriber about moving from 50 mg to 100 mg if the response was partial.
- If four to eight properly conducted attempts at the maximum tolerated dose fail, that is the point to discuss switching molecules or moving to second-line options.
Side effects and risks
Most side effects come directly from blocking PDE5 in tissues other than the penis. They are dose-related, usually mild, and often fade after the first few uses.
| Effect | Roughly how common | What helps |
|---|---|---|
| Headache | 1 in 6 | Hydration and a simple analgesic; often improves with repeated use or a lower dose. |
| Facial flushing | 1 in 10 | Harmless. Tends to lessen over time. |
| Nasal congestion | 1 in 10 | Saline spray. Avoid stacking decongestants if you have high blood pressure. |
| Indigestion / reflux | 1 in 14 | Avoid lying flat straight afterwards; discuss antacids with your pharmacist. |
| Blue tinge or light sensitivity | About 1 in 30 | From weak PDE6 blockade in the retina. Temporary. More common at 100 mg. |
| Back or muscle ache | Uncommon | More typical of tadalafil than sildenafil. |
Rare but serious
- Non-arteritic anterior ischaemic optic neuropathy (NAION) — sudden painless vision loss in one eye. Very rare. Risk appears higher in people who already have a 'crowded' optic disc.
- Sudden sensorineural hearing loss, sometimes with tinnitus or dizziness.
- Symptomatic hypotension, particularly when combined with alpha-blockers or in people who are dehydrated.
Who should not take it
Use only with specialist advice
- Alpha-blockers for prostate symptoms or blood pressure (tamsulosin, doxazosin, alfuzosin) — start at the lowest ED dose once you are stable on the alpha-blocker, and separate the doses.
- Strong CYP3A4 inhibitors — ketoconazole, itraconazole, ritonavir, clarithromycin. These raise sildenafil levels sharply and usually require a 25 mg starting dose.
- Recent heart attack, stroke or life-threatening arrhythmia (within roughly six months), unstable angina, uncontrolled hypertension or significant hypotension.
- Severe liver disease or end-stage kidney disease — lower starting dose.
- Retinitis pigmentosa or a previous episode of NAION.
- Conditions predisposing to priapism — sickle cell disease, multiple myeloma, leukaemia.
One more thing worth saying plainly: sex is exercise. If you get chest pain, breathlessness or palpitations climbing two flights of stairs, get your heart assessed before you start treating the erection. See ED and cardiovascular disease.
What it costs
Sildenafil's US patent expired in December 2017. Generic tablets are now one of the clearest examples of how far a drug price can fall once competition arrives.
| Route | Typical price | Notes |
|---|---|---|
| Generic, retail pharmacy with a discount card | $5–$40 for 30 tablets | Often the cheapest route by a wide margin. Prices vary a lot between pharmacies in the same town. |
| Generic via telehealth subscription | $10–$30 per month | Convenience and an included consultation; you are paying for the service, not the molecule. |
| Brand-name Viagra | $70–$100 per tablet | Chemically identical active ingredient to the generic. |
Most US insurance plans cover ED medication poorly or not at all, and many that do cover it impose quantity limits. Medicare Part D generally excludes drugs used for erectile dysfunction. Assume you are paying cash, and shop accordingly.
How to get it
Sildenafil is prescription-only everywhere in the United States. You have three realistic routes:
- Your regular clinician
- The best option if you have other health conditions, take several medications, or have not had a cardiovascular check recently. They can also investigate the cause rather than just the symptom.
- A telehealth service
- Fast, private, and reasonable for otherwise healthy men. Quality varies — see our reviews of the main services and our guide to buying safely.
- A urologist
- Worth going straight there if oral medication has already failed, if you have had prostate surgery, or if you have penile curvature or pain.
Frequently asked questions
How long does sildenafil take to work?
How long does it last?
Will it work if I am not aroused?
Can I take it every day?
Is 100 mg simply better than 50 mg?
Does it stop working over time?
Can I drink alcohol with it?
What this is based on
We link to primary sources — clinical guidelines, regulator publications and peer-reviewed research — wherever possible. Links open on third-party sites we do not control.
- [01]U.S. Food and Drug Administration. Drugs@FDA — approved drug products and prescribing information.
- [02]American Urological Association. Erectile Dysfunction: AUA Guideline (2018).
- [03]National Institute of Diabetes and Digestive and Kidney Diseases. Treatment for Erectile Dysfunction.
- [04]Mayo Clinic. Erectile dysfunction — symptoms and causes.