# Sildenafil (Viagra): How It Works, Dosing, Cost and Side Effects

> Sildenafil is the most-prescribed erectile dysfunction medication in the world and the one most people try first. Here is what it does, how to take it properly, what it realistically costs, and when it is not safe.

- **Source:** ED Treatment Guides (https://www.edtreatmentguides.com/treatments/sildenafil)
- **Last updated:** 2025-11-18
- **Type:** treatment
- **Category:** Oral medication
- **Prescription:** Prescription only
- **Evidence:** Strong evidence
- **Onset:** 30–60 minutes
- **Duration:** 4–6 hours
- **Typical US cash cost:** $5–$40 per month (generic) · $70–$100 per tablet (brand Viagra)
- **Best for:** A first attempt at treatment for most causes of ED; People who want a low-cost, well-understood option; Planned rather than spontaneous sex
- **Not for:** Anyone taking nitrates or riociguat; People who want a treatment that lasts more than a few hours; People with unstable cardiac disease until cleared by a clinician
- **Common side effects:** Headache; Facial flushing; Nasal congestion; Indigestion or reflux; Blue-tinged or light-sensitive vision; Muscle aches

## Key points

- Sildenafil does not create an erection on its own — it amplifies the body's normal response to sexual arousal by keeping blood vessels in the penis relaxed for longer.
- Take it 30–60 minutes before sex on a relatively empty stomach. A heavy or fatty meal can delay it by an hour or more.
- Roughly two-thirds of men respond, but the figure is lower after prostate surgery or with long-standing diabetes.
- Generic sildenafil is inexpensive. Paying brand-name prices for the same molecule is almost never necessary.
- It must never be combined with nitrates. That interaction is the single most important thing to know about this drug class.

## 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.

> **This is the part people get wrong**
>
> Sildenafil is an amplifier, not a switch. Without sexual stimulation, nothing happens. Men who take a tablet, sit and wait for an effect, and conclude the drug 'didn't work' are the single largest group of apparent treatment failures.

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:

**Approximate response rates by underlying cause**

| 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. |

_Figures are approximate ranges from published trial and registry data, presented to set expectations — not to predict your individual result._

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

**Standard adult dosing for erectile dysfunction**

| 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. |

_Maximum frequency is **once in any 24-hour period**. Taking more than the maximum dose increases side effects without improving results._

### 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

1. Confirm the basics: taken on a light stomach, enough time before sex, and with genuine sexual stimulation.
2. Repeat the same dose several times before changing anything. Performance anxiety on the first attempt is extremely common and resolves with repetition.
3. Ask your prescriber about moving from 50 mg to 100 mg if the response was partial.
4. 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](https://www.edtreatmentguides.com/treatments).

## 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.

**What to expect**

| 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. |

> **Get emergency care**
>
> An erection lasting longer than **4 hours** (priapism) is a medical emergency and needs an emergency department, not a phone call. Sudden loss of vision or hearing after a dose also needs same-day medical attention.

### 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.

> **Note**
>
> Sildenafil does not cause prostate cancer, does not affect fertility, does not increase penis size, and does not increase libido. It treats the mechanics of an erection and nothing else.

## Who should not take it

> **Absolute contraindication**
>
> Never combine any PDE5 inhibitor with nitrate medication — nitroglycerin tablets or spray, isosorbide mononitrate or dinitrate, or recreational 'poppers' (amyl/butyl nitrite). The combination can cause a sudden, dangerous drop in blood pressure. It is also contraindicated with riociguat (Adempas). If you take nitrates for chest pain, tell every prescriber before starting an ED medication. [Read the full safety guide](https://www.edtreatmentguides.com/guides/pde5-safety).

### 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.

> **Nitrate timing after a dose**
>
> If you take sildenafil and later need emergency nitrate treatment for chest pain, the standard advice is that at least **24 hours** must have passed. Tell the paramedic or emergency clinician exactly what you took and when — this is not a moment for embarrassment.

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](https://www.edtreatmentguides.com/causes/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.

**Typical US cash prices**

| 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. |

_Prices are US cash estimates as of the update date at the top of this page and change frequently. Always check current pricing before you buy._

> **Ask about tablet strength, not just quantity**
>
> Because pricing is often per tablet rather than per milligram, the same monthly spend can buy very different amounts of medication depending on the strength prescribed. This is a legitimate conversation to have with your prescriber and pharmacist — but never split, double or otherwise adjust tablets on your own. See our [cost and insurance guide](https://www.edtreatmentguides.com/guides/cost-and-insurance).

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](https://www.edtreatmentguides.com/reviews) and our [guide to buying safely](https://www.edtreatmentguides.com/guides/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.

> **Never buy without a prescription**
>
> Sites that sell sildenafil with no prescription are operating illegally. The FDA has repeatedly found such products to contain the wrong dose, no active ingredient, or contaminants. See [spotting counterfeit medication](https://www.edtreatmentguides.com/guides/counterfeit-medication).

## Frequently asked questions

### How long does sildenafil take to work?

Usually 30 to 60 minutes, and faster on an empty stomach. Some men notice an effect within 20 minutes; a heavy meal can push it past 90 minutes.

### How long does it last?

The useful window is about 4 to 6 hours for most men. That does not mean a continuous erection — it means the drug is available to help when arousal occurs during that period.

### Will it work if I am not aroused?

No. Sildenafil amplifies the body's own response to sexual stimulation. Without arousal it does essentially nothing.

### Can I take it every day?

Sildenafil is licensed for use as needed, no more than once in 24 hours. If you want something you take daily, [tadalafil has a licensed low-dose daily regimen](https://www.edtreatmentguides.com/treatments/tadalafil).

### Is 100 mg simply better than 50 mg?

No. Higher doses raise the ceiling for men with a partial response, but they also raise the rate of headache, flushing and visual effects. The right dose is the lowest one that works reliably for you.

### Does it stop working over time?

The drug does not lose potency, but the underlying condition can progress — particularly with diabetes or vascular disease. If something that used to work stops working, that is a reason to be re-assessed rather than simply to take more.

### Can I drink alcohol with it?

A drink or two is generally fine. Heavy drinking impairs erections directly, adds to the blood-pressure-lowering effect and makes dizziness more likely.

## Sources

- U.S. Food and Drug Administration. [Drugs@FDA — approved drug products and prescribing information](https://www.accessdata.fda.gov/scripts/cder/daf/)
- American Urological Association. [Erectile Dysfunction: AUA Guideline](https://www.auanet.org/guidelines-and-quality/guidelines/erectile-dysfunction-(ed)-guideline) (2018)
- National Institute of Diabetes and Digestive and Kidney Diseases. [Treatment for Erectile Dysfunction](https://www.niddk.nih.gov/health-information/urologic-diseases/erectile-dysfunction/treatment)
- Mayo Clinic. [Erectile dysfunction — symptoms and causes](https://www.mayoclinic.org/diseases-conditions/erectile-dysfunction/symptoms-causes/syc-20355776)

## Related pages

- [Tadalafil (Cialis): The 36-Hour Option, Daily Dosing and Costs](https://www.edtreatmentguides.com/treatments/tadalafil)
- [Viagra vs Cialis: Which One Should You Try First?](https://www.edtreatmentguides.com/compare/viagra-vs-cialis)
- [PDE5 Inhibitor Safety: Interactions, Contraindications and Emergencies](https://www.edtreatmentguides.com/guides/pde5-safety)
- [ED Medication Dosage Guide](https://www.edtreatmentguides.com/guides/dosage-guide)

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_This page is general health information, not medical advice. It cannot account for your own history, medications or risk factors. Talk to a licensed clinician before starting, stopping or changing any treatment._

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