Medication guide

Sildenafil (Viagra): how it works, timing, food, and generics

Sildenafil is the active ingredient in Viagra and in many FDA-approved generic tablets used for erectile dysfunction. For most adults, U.S. labeling starts at 50 mg, taken as needed about one hour before sexual activity. The dose may be taken anywhere from 30 minutes to 4 hours beforehand, and not more than once a day. A prescriber may lower it to 25 mg or raise it to 100 mg. Those numbers are labeling ranges, not a plan to set for yourself.

What the tablet actually does

An erection depends on blood staying in the erectile tissue. Sexual stimulation releases nitric oxide from nerves in the penis. Nitric oxide raises cyclic GMP inside smooth muscle, the muscle relaxes, and the corpora cavernosa fill. The enzyme PDE5 breaks cyclic GMP down. Sildenafil slows that breakdown, so the signal from stimulation lasts longer.

Two limits follow from that mechanism. First, without stimulation there is little nitric oxide signal to protect, so sildenafil does not cause an erection on its own and it is not an aphrodisiac. Second, the same cyclic GMP pathway exists in blood vessels elsewhere. Nitrates increase nitric oxide throughout the circulation. Blocking PDE5 at the same time removes a brake, which is why the blood-pressure drop can be severe.

Sildenafil tablets are rapidly absorbed. In the fasted state, peak blood levels arrive in about 30 to 120 minutes, with a median near 60 minutes. Absolute bioavailability averages about 41 percent. The drug and its main active metabolite both have a terminal half-life of about 4 hours. In a laboratory study of rigidity, an effect was still present at 4 hours but was weaker than at 2 hours. That is the practical “about four hour” window people mean. It is not an erection that lasts four hours. An erection that is still present at four hours is a separate emergency, covered on the priapism page.

Food, and the empty-stomach advice

The patient label says sildenafil may be taken with or without food. The clinical pharmacology section is more specific. When a dose is taken with a high-fat meal, absorption slows: the time to peak concentration is delayed by a mean of about 60 minutes, and the peak concentration falls by a mean of about 29 percent. A heavy, fatty dinner can therefore turn a planned one-hour onset into something closer to two hours, with a lower peak.

“Take it on an empty stomach” is a timing tactic, not a safety rule that the dose becomes unsafe if you have eaten. A lighter meal interferes less than a high-fat one. If dinner is the setting, many clinicians talk through taking the dose earlier, choosing a lighter plate, or discussing a medicine with less food effect, such as tadalafil. That switch is a prescribing decision. Details of meals, alcohol, and grapefruit are on the interactions page.

Doses that appear on the adult label

Erectile-dysfunction tablets are supplied at 25 mg, 50 mg, and 100 mg. For most patients the recommended dose is 50 mg as needed. Based on effect and side effects, the prescriber may increase to a maximum of 100 mg or decrease to 25 mg. The maximum recommended frequency is once per day.

Labeling points toward a 25 mg start in several situations, because blood levels run higher:

• Age over 65
• Hepatic impairment
• Severe renal impairment (creatinine clearance under 30 mL/min)
• Stable alpha-blocker therapy, because the two drug classes both lower blood pressure
• Strong CYP3A4 inhibitors such as ketoconazole, itraconazole, or saquinavir, and the antibiotic erythromycin

Ritonavir is called out separately. It raised sildenafil exposure about 11-fold in studies. The label’s maximum for someone treated with ritonavir is 25 mg, and not more than 25 mg in a 48-hour period. Other HIV medicines and boosters need a pharmacist’s review. Do not apply these caps on your own by splitting tablets from a different prescription.

Sildenafil is also sold for pulmonary arterial hypertension, including under the brand Revatio, often as a 20 mg tablet taken on a different schedule. That is a different indication. Using those tablets to construct an erectile-dysfunction dose is a common shortcut and a common error. Take the product and schedule your prescriber wrote.

Side effects people actually feel

In trials, the reactions that showed up more often as the dose rose included headache, flushing, indigestion, nasal congestion, and abnormal vision (often a blue tinge or light sensitivity). Back pain and muscle aches are more characteristic of tadalafil than of sildenafil, though they can occur. Most of these fade as the dose wears off. A lower dose, a different PDE5 inhibitor, or treating the headache without adding another interacting drug are conversations for the prescriber, not experiments.

Stop the drug and get care the same day for sudden vision loss in one or both eyes, or a sudden drop in hearing, sometimes with ringing or dizziness. Sudden vision loss can be NAION, a problem of blood flow to the optic nerve. The label says a causal link is not proven, and many people who developed it already had vascular risk, but the instruction to patients is still to stop and be examined. People with a prior episode of NAION, or a crowded optic disc, need that history in the chart before a first prescription.

Rare cases of erection lasting longer than 4 hours, and priapism (a painful erection longer than 6 hours), have been reported. The instruction is immediate medical care at 4 hours, because delay can scar erectile tissue. Extra caution applies with Peyronie’s disease, other anatomic deformity, sickle cell disease, multiple myeloma, and leukemia. There are no controlled safety data for sildenafil in sickle cell disease.

Who should not start with a tablet

Sexual activity itself raises cardiac work. Sildenafil is not for men whose clinicians have said sexual activity is unsafe. A single 100 mg dose lowered sitting blood pressure by a mean maximum of about 8/5 mm Hg in healthy volunteers. That is modest for many people and unsafe for some, including people with severe autonomic dysfunction or obstruction to left ventricular outflow, such as significant aortic stenosis.

Tell the prescriber about nitrates, riociguat, alpha-blockers (tamsulosin, doxazosin, and others), HIV medicines, azole antifungals, erythromycin, clarithromycin, other blood-pressure drugs, and any prior sudden vision or hearing loss. Also say if you have had an erection that would not go down, and whether erections are absent only with a partner or also on waking. That pattern helps separate a performance-anxiety problem from a vascular one. Both can exist together. The causes page explains the distinction, and the prescription pathway explains what the visit is for.

Questions this page can answer

Does generic sildenafil work as well as Viagra?

An FDA-approved generic contains the same active ingredient and has to perform like the brand in bioequivalence testing. Inactive ingredients and appearance can differ. A product that is not an FDA-approved medicine, including a supplement with hidden sildenafil, is not a generic.

Can I take it after dinner?

Yes, the label allows food. A high-fat meal delays the peak by about an hour on average and lowers it by about 29 percent. If timing matters that evening, a lighter meal is the practical adjustment.

What if it “doesn’t work” the first time?

Trials measured the response with sexual stimulation, usually around an hour after the dose. A heavy meal, too little stimulation, alcohol, or anxiety can make a first try look like failure. Dose changes belong to the prescriber. Taking a second tablet the same day exceeds the labeled maximum.

Is 20 mg the same as 25 mg?

No. Twenty milligrams is a strength used for pulmonary arterial hypertension, on its own schedule. Erectile-dysfunction labeling uses 25, 50, and 100 mg. Do not substitute one product for the other.

Sources

• Sildenafil tablets prescribing information, DailyMed / FDA, including dosage, nitrate contraindication, food effect on Tmax and Cmax, and priapism counseling.
• FDA generic-drug approval standard: same active ingredient, strength, dosage form, and bioequivalence.

Related: compare all four PDE5 inhibitors, tadalafil, nitrates and the heart.