Sildenafil & Viagra Guide: Prescription, OTC, and Online Options for Patients
Sildenafil citrate is a compound approved by the U.S. Food and Drug Administration (FDA) in 1998 under the brand name Viagra. It belongs to a class called PDE5 inhibitors. Since 2017, multiple generic versions have been widely available in the United States, making it one of the most studied and documented compounds in its category. This guide summarizes what U.S. patients typically ask about sildenafil: how it works, what a provider visit involves, available forms, dosing ranges, and cost context.
Who this guide is for: Adults in the United States who have questions about sildenafil and want factual, sourced background before or after speaking with a licensed healthcare provider. This is not a substitute for personalized medical guidance.
What Sildenafil Is and How It Works
Sildenafil citrate is a selective inhibitor of phosphodiesterase type 5 (PDE5), an enzyme found in smooth muscle tissue. In the context of erectile dysfunction (ED), the drug works by enhancing the effect of nitric oxide (NO), a chemical naturally released during sexual stimulation. Nitric oxide activates an enzyme that produces cyclic guanosine monophosphate (cGMP). cGMP relaxes smooth muscle in the corpus cavernosum of the penis, allowing increased blood flow.
Sildenafil inhibits PDE5, the enzyme that degrades cGMP. By slowing cGMP breakdown, sildenafil prolongs the vasodilatory response. Importantly, it does not independently produce an erection; sexual stimulation is required for the mechanism to activate.
The FDA also approved a separate sildenafil formulation (Revatio, 20 mg) for pulmonary arterial hypertension (PAH), a distinct cardiovascular indication. Dosing and clinical context differ substantially between that use and ED.
Prescription Context in the United States
Sildenafil for erectile dysfunction is an FDA-approved compound that requires a valid note from a licensed U.S. healthcare provider. It is not available over the counter without one. This requirement exists because sildenafil has clinically significant interactions with several other drug classes—particularly nitrates used in cardiac care—and contraindications that a provider must assess before authorizing it.
Patients obtain a provider note through several pathways:
- In-person primary care visit — a standard physical and health history review with a physician, nurse practitioner, or physician assistant
- Urology or men’s health specialist — often appropriate if ED may relate to a structural or hormonal cause
- Telehealth consultation — several state-licensed telehealth platforms allow an asynchronous or live video consultation, after which a provider can send a note electronically to a pharmacy of the patient’s choice
Important: Sildenafil is contraindicated with nitrate compounds (such as nitroglycerin, isosorbide mononitrate, isosorbide dinitrate) due to risk of severe hypotension. It is also contraindicated with riociguat. Patients using these compounds should inform their provider before discussing sildenafil.
Brand Name vs. Generic: What FDA Approval Means
Pfizer’s Viagra (100 mg sildenafil citrate) was the first-approved branded formulation in the U.S., receiving FDA approval in March 1998. The drug’s compound patent exclusivity expired in December 2017. After that date, the FDA authorized generic manufacturers to produce sildenafil citrate tablets.
FDA bioequivalence standards require that approved generics deliver the same active ingredient in the same amount, via the same route, and within an equivalent absorption window as the reference listed drug. Excipients (inactive ingredients) may differ, but the pharmacokinetic profile must fall within FDA’s defined equivalence range. For patients, this means an FDA-approved generic sildenafil is held to the same standard as the brand name.
Generic sildenafil is available from multiple manufacturers in 25 mg, 50 mg, and 100 mg tablet strengths. See our Generic Availability page for information on how to identify FDA-approved versions.
Dosing Overview
The labeled doses for sildenafil in erectile dysfunction are 25 mg, 50 mg, and 100 mg. Most U.S. providers start patients at 50 mg, to be taken approximately 30 to 60 minutes before anticipated sexual activity. Based on individual response and tolerability, the dose may be adjusted in either direction.
| Dose | Typical use context | Max per 24 hours |
|---|---|---|
| 25 mg | Starting dose for older adults, those on certain interacting compounds, or when 50 mg causes side effects | 25 mg |
| 50 mg | Standard starting dose for most adults; most commonly studied dose | 50 mg |
| 100 mg | Maximum labeled dose; used when 50 mg is inadequate and well tolerated | 100 mg |
Sildenafil should not be taken more than once per day. For more detail on timing, food interactions, and special populations, see the Dosing Guide.
Common Side Effects and Safety Notes
Clinical trial data published with sildenafil’s approval identified several common adverse effects. The most frequently reported were headache (occurring in roughly 16% of participants at 50 mg in pivotal trials), flushing (10%), dyspepsia (7%), and nasal congestion (4%). Visual disturbances—typically described as a transient bluish tint or increased light sensitivity—were reported in about 3% of patients at standard doses.
These effects are generally dose-dependent and transient. Serious adverse events are rare at labeled doses in appropriately screened patients but have occurred, particularly in individuals with underlying cardiovascular disease or those using interacting compounds.
- Sildenafil can cause a transient drop in blood pressure; caution applies in patients with low resting blood pressure or recent stroke
- Rare cases of non-arteritic anterior ischemic optic neuropathy (NAION) have been reported; patients with prior NAION or certain anatomical eye conditions should discuss this with their provider
- Priapism (prolonged erection exceeding 4 hours) is rare but requires immediate medical attention
Comparing PDE5 Inhibitors: Sildenafil, Tadalafil, Vardenafil
Sildenafil is one of three widely used PDE5 inhibitors approved by the FDA for ED. The others are tadalafil (Cialis and generics) and vardenafil (Levitra and generics). All three share the same mechanism but differ in half-life, onset speed, and interaction profiles.
| Compound | Onset (approx.) | Duration (approx.) | Food effect |
|---|---|---|---|
| Sildenafil | 30–60 min | 4–5 hours | High-fat meal delays absorption |
| Tadalafil | 30–45 min | Up to 36 hours | Minimal effect |
| Vardenafil | 25–60 min | 4–5 hours | High-fat meal may reduce peak levels |
Choice among these compounds is typically guided by patient preference, frequency of use, underlying health conditions, and tolerability. Providers weigh these factors during a consultation.
Talking to a Healthcare Provider
A provider visit for ED typically covers medical history, current compounds (both prescription and OTC), cardiovascular status, blood pressure, and any symptoms that might indicate an underlying cause for ED such as diabetes, hypogonadism, or vascular disease. This is clinically relevant: ED can be an early marker of cardiovascular risk in some patients.
Patients who have had a satisfactory experience with sildenafil and want a refill can often do so through a follow-up with their primary care provider, or through telehealth for established patients in their state.