Viagra (sildenafil): uses, dosage, side effects and refills

Sildenafil is the oldest PDE5 inhibitor on the US market. The FDA approved Pfizer's Viagra for erectile dysfunction on 27 March 1998 (Drugs@FDA, NDA 020895); the brand now belongs to Viatris. The same molecule at 20 mg, sold as Revatio since 2005, treats pulmonary arterial hypertension (PAH). Generic sildenafil in all of these strengths is widely sold. ClinCalc's DrugStats estimates 3.5 million prescriptions in 2024, ranking it 160th.

Sildenafil is short-acting compared with tadalafil. Both it and its active metabolite have terminal half-lives of about 4 hours (Viagra label, 12.3), so it is used as needed before sex rather than daily for ED. In the label's RigiScan study, effects lasted up to 4 hours but were weaker than at 2 hours.

Like other PDE5 inhibitors, it does not create desire or an erection on its own; sexual stimulation is still required. It does not prevent pregnancy or sexually transmitted infections (MedlinePlus).

This page focuses on sildenafil for ED. Revatio for PAH is managed by lung or heart specialists at different doses.

FDA-approved uses

• Erectile dysfunction (Viagra label, section 1).
• Pulmonary arterial hypertension (WHO Group I) in adults, to improve exercise ability and delay clinical worsening, and in children 1 to 17 years (Revatio label, section 1).

What the ED trials showed (Viagra label, section 14)

Sildenafil was tested in 21 placebo-controlled trials in more than 3,000 men aged 19 to 87, and beat placebo in all 21. In fixed-dose trials, 63%, 74% and 82% of men on 25, 50 and 100 mg reported improved erections, compared with 24% on placebo. In a trial limited to men whose ED stemmed from diabetes, 48% of intercourse attempts succeeded on sildenafil versus 12% on placebo. Results are group averages; underlying cause matters.

Off-label uses

MedlinePlus notes other uses may be prescribed. Any use outside ED and PAH is off-label; ask your prescriber about the evidence.

What it does not do

• It does not increase libido.
• It does not treat low testosterone, which has its own therapy (testosterone).
• It is not approved for women.

The Viagra label says evaluating ED should include a complete medical assessment to find underlying causes (5.1).

During sexual stimulation, nerves and blood vessel lining release nitric oxide, which raises cyclic GMP (cGMP) in the erectile tissue. cGMP relaxes smooth muscle so blood flows in and an erection forms. The enzyme PDE5 then breaks cGMP down. Sildenafil blocks PDE5, so cGMP builds up more when stimulation happens (Viagra label, 12.1).

In the lungs, blocking PDE5 relaxes blood vessels, the basis for Revatio in PAH.

Pharmacokinetics (Viagra label, 12.3)

• Absorbed quickly, with peak levels at a median of 60 minutes on an empty stomach (range 30 to 120 minutes).
• A high-fat meal delays the peak by about 60 minutes and lowers it by 29%.
• About 41% of a dose reaches the blood; the liver clears it mainly through CYP3A4.
• Half-life about 4 hours.

Blood pressure and vision. In healthy men, sildenafil lowered supine blood pressure by an average of 8.4/5.5 mm Hg (5.1). It is only about 10 times more selective for PDE5 than for PDE6, a retinal enzyme (12.1), which explains the blue tinge or light sensitivity some people notice at higher doses.

Why nitrates are dangerous. Nitrates release nitric oxide; with PDE5 blocked, the combined blood vessel relaxation can cause severe, even fatal, hypotension.

Do not take sildenafil if you (Viagra label, section 4):

• use nitric oxide donors: organic nitrates or nitrites in any form, regularly or occasionally. That includes nitroglycerin tablets, spray, patches or ointment; isosorbide mononitrate or dinitrate; and "poppers" such as amyl nitrite or butyl nitrite;
• take riociguat or another guanylate cyclase stimulator;
• are allergic to sildenafil (Viagra or Revatio).

Tell your prescriber about:

• Heart disease. ED drugs should generally not be used where sexual activity is inadvisable for cardiovascular reasons (5.1). Mention recent heart attack, stroke, unstable angina, heart failure, aortic stenosis, or very low or uncontrolled high blood pressure.
• Alpha blockers such as tamsulosin, doxazosin or alfuzosin, and other blood pressure medicines including amlodipine (5.5).
• HIV medicines, especially ritonavir, and antifungals or antibiotics such as ketoconazole, itraconazole or erythromycin.
• Past sudden vision loss (NAION), a crowded optic disc, or retinitis pigmentosa (5.3).
• Conditions that raise priapism risk or penile curvature (5.2).
• Bleeding disorders or active stomach ulcers (5.8).
• Liver disease, severe kidney disease or age over 65, each a reason to consider starting at 25 mg.
• Revatio or any other sildenafil product, which must not be doubled up (5.7).

Ask: "Am I healthy enough for sex, and what should I tell an emergency team if I get chest pain after taking it?"

For ED (Viagra label, section 2; MedlinePlus)

• The usual dose is 50 mg, taken as needed about 1 hour before sexual activity. It can be taken anywhere from 30 minutes to 4 hours beforehand.
• Your prescriber may raise the dose to 100 mg or lower it to 25 mg.
• Take it no more than once a day.
• It works with or without food, but a high-fat meal slows it, delaying the effect by about an hour.
• Sexual stimulation is needed for it to work.

Starting lower (25 mg) is considered for people over 65, with liver impairment, with severe kidney impairment, or taking erythromycin or a strong CYP3A4 inhibitor (2.4, 2.5). People on ritonavir should take no more than 25 mg in 48 hours.

With an alpha blocker, you should be stable on the alpha blocker first and start sildenafil at 25 mg (2.3).

For PAH (Revatio): 20 mg three times a day, adjusted by a specialist (Revatio label, 2.1). Do not use Revatio tablets for ED alongside Viagra.

Safety habits

• Keep a note of when you took sildenafil. If chest pain occurs, call 911 and tell responders, because the label says it is unknown when nitrates can safely be given after sildenafil (4.1).
• Stop and seek help for sudden vision or hearing loss, or an erection lasting more than 4 hours.

Your prescriber chooses the dose. These figures are from the Viagra and Revatio labels.

• Age: men 65 and over had about 84% higher sildenafil blood levels than young men, so 25 mg is the suggested start (Viagra label, 8.5).
• Kidneys: mild or moderate impairment needs no change; severe impairment (creatinine clearance under 30 mL/min) roughly doubles exposure. Temi's creatinine clearance calculator explains the measure.
• Liver: Child-Pugh A or B raised exposure by 85%; consider 25 mg (8.7).
• Frequency: for ED, once a day is the maximum.
• No combinations: do not use sildenafil with tadalafil, vardenafil, avanafil or Revatio for ED (5.7).

For erectile dysfunction, MedlinePlus points out that a missed dose is unlikely, because sildenafil is taken only when needed rather than on a schedule. For PAH (Revatio), take the missed dose once you remember unless the next is close, then return to the usual times without doubling up.

Healthy volunteers given single doses up to 800 mg had the same side effects as at normal doses, only more often and more severely (Viagra label, 10). At 200 to 800 mg, some had low blood pressure, fainting and prolonged erections (5.6). Dialysis does not help. Call Poison Help at 1-800-222-1222 for an accidental overdose, and 911 for fainting, chest pain or an erection lasting over 4 hours.

• Nitrates of any kind, including nitroglycerin, isosorbide products and "poppers".
• Riociguat.
• Taking it more than once a day, or with tadalafil or Revatio.
• A heavy, high-fat meal right before, which delays the effect.
• Grapefruit juice in large amounts: sildenafil is a CYP3A4 substrate; ask your pharmacist.
• Driving if your vision is tinged or blurred after a dose.
• Ignoring chest pain during sex: stop and get emergency help.

The Viagra label states sildenafil is not indicated for use in females. Rats and rabbits given doses up to 16 and 32 times the maximum human dose during organogenesis showed no birth defects (Viagra label, 8.1). Sildenafil for PAH during pregnancy is a specialist decision under the Revatio label.

Male fertility: single 100 mg doses had no effect on sperm motility or shape, and less than 0.001% of a dose reaches semen (Viagra label, 12.2 and 12.3).

Viagra and its generics are stored at 25°C (77°F), with short excursions from 15°C to 30°C acceptable (Viagra label, 16). Do not keep a spare tablet loose in a wallet or car glove box where heat and humidity build. Children should never be able to reach them; dispose of extras through a pharmacy kiosk.

Related medicines, each with its own page:

• Tadalafil: a longer-acting PDE5 inhibitor that can be taken daily.
• Nitroglycerin: a nitrate for angina that must never be combined with sildenafil.
• Isosorbide mononitrate: a daily nitrate, also contraindicated.
• Tamsulosin: an alpha blocker that requires a low sildenafil starting dose.
• Testosterone: treats low testosterone, another cause of sexual symptoms.
• Finasteride: can cause sexual side effects of its own.

Sildenafil for erectile dysfunction is non-controlled, and Temi physicians can prescribe it online. Renewing a dose that already works is a refill review at $15, waived for Temi Care members (usetemi.com/prescription/refills, checked 5 Oct 2026). A first prescription requires a $79 new prescription review (usetemi.com/get-prescribed, checked 5 Oct 2026), with screening for nitrates, heart conditions and alpha blockers. Revatio for PAH is specialist-managed, so a Temi physician decides case by case and may decline. You hear back within 48 hours, and your pharmacy fills it.