Sildenafil: uses, side effects and safety
Sildenafil (Viagra) is a phosphodiesterase type 5 (pde5) inhibitor. It has no US federal controlled-substance schedule. It is prescription only (rx) and remains on the US market.
What is Sildenafil?
Sildenafil is the generic name for Viagra, and belongs to the Phosphodiesterase type 5 (PDE5) inhibitor class.
| Generic name | sildenafil |
|---|---|
| Brand name(s) | Viagra |
| Drug class | Phosphodiesterase type 5 (PDE5) inhibitor |
| US legal status | No US federal schedule · Prescription only (Rx) |
| Route | Oral |
| Prescription required | Yes |
| Elimination half-life | Approximately 4 hours (both sildenafil and the N-desmethyl metabolite) |
| Time to peak concentration | 1 |
| Metabolism | Cleared predominantly by CYP3A4 (major route) and CYP2C9 (minor route) hepatic microsomal isoenzymes. Major circulating metabolite is N-desmethyl sildenafil, further metabolized. |
What is Sildenafil used for?
Sildenafil is approved by the FDA for the following uses. Approval is granted per indication, and a specific product may not be approved for all of them.
- Treatment of erectile dysfunction (ED)
How does Sildenafil work?
Enhances the effect of nitric oxide by inhibiting phosphodiesterase type 5 (PDE5), which is responsible for degradation of cGMP in the corpus cavernosum. Increased cGMP produces smooth muscle relaxation and inflow of blood. Approximately 4,000-fold more selective for PDE5 than PDE3. No effect in the absence of sexual stimulation..
It is metabolised primarily by Cleared predominantly by CYP3A4 (major route) and CYP2C9 (minor route) hepatic microsomal isoenzymes. Major circulating metabolite is N-desmethyl sildenafil, further metabolized..
The elimination half-life is roughly Approximately 4 hours (both sildenafil and the N-desmethyl metabolite). Longer half-lives mean the drug and its active metabolites persist in the body for longer after the last dose.
Common side effects of Sildenafil
These are the side effects people report most often. Many diminish after the body adjusts, usually within one to two weeks.
- Headache
- Flushing
- Dyspepsia
- Abnormal vision (including color vision changes)
- Nasal congestion
- Back pain
- Myalgia
- Nausea
- Dizziness
- Rash
What does Sildenafil interact with?
Interactions matter because two safe medicines can become dangerous together. The entries below are the ones most likely to cause harm.
- Nitrates / nitric oxide donors (organic nitrates or nitrites, in any form): CONTRAINDICATED - potentiates hypotensive effects and can cause fatal hypotension
- Alpha blockers: initiate Viagra at 25 mg once the patient is stable on alpha-blocker therapy
- Antihypertensives: potentiates the hypotensive effect
- Ritonavir: do not exceed a maximum single dose of 25 mg in a 48 hour period (11-fold increase in AUC)
- Strong CYP3A4 inhibitors (ketoconazole, itraconazole, saquinavir) and erythromycin: consider a 25 mg starting dose
- High-fat meal: delays Tmax by a mean of 60 minutes and reduces Cmax by a mean of 29%
Use our drug interaction checker before combining any two prescriptions.
Serious side effects and warning signs
Seek emergency care for any of these
- Life-threatening hypotension with nitrates - potentially fatal (contraindicated)
- Priapism and prolonged erections, including those lasting more than 4 hours
- Sudden vision loss in one or both eyes (non-arteritic anterior ischemic optic neuropathy)
- Sudden hearing loss
- Myocardial infarction, stroke and transient ischemic attack
- Seizures
Suspected overdose or a bad reaction should be reported to Poison Control on 1-800-222-1222. That is staffed around the clock and is free.
Legal status of Sildenafil
Sildenafil has no US federal controlled-substance schedule. An unscheduled medicine is not an over-the-counter medicine: it remains prescription-only, and individual states may regulate it independently. State rules are covered in our state-by-state section.
Statutory reference: Not listed in 21 CFR 1308.11 through 1308.15 (Schedules II-V), verified against the eCFR text current as of 2026-09-01. Vardenafil and tadalafil are likewise absent..
Important note
Nitrate co-administration is the standout fatal interaction and is contraindicated, not merely cautioned. onsetHours 1 is the labelled administration time (approximately 1 hour before sexual activity; may be taken 30 minutes to 4 hours before), corroborated by MedlinePlus. peakHours 1 is the label's stated median Tmax of 60 minutes (maximum observed plasma concentrations reached within 30 to 120 minutes in the fasted state). schedule is null because sildenafil does not appear anywhere in 21 CFR 1308.11-1308.15 - a verified absence, not a missing lookup. This entry covers Viagra (ED) only; sildenafil tablets for pulmonary arterial hypertension are a separate product and indication.
Can you get Sildenafil without a prescription?
Short answer: no. Sildenafil is a prescription medication. Sites advertising it without one are either operating illegally or selling something other than what they claim. We explain the legal routes, the counterfeit problem and how to check a source in detail.
Read the full answer: Sildenafil without a prescription →
Frequently asked questions
What is Sildenafil used for?
Treatment of erectile dysfunction (ED)
Is Sildenafil a controlled substance?
Sildenafil has no US federal controlled-substance schedule. That is not the same as being available over the counter: it is still a prescription-only medicine, and a number of individual states regulate it separately.
What are the common side effects of Sildenafil?
The most frequently reported side effects are headache, flushing, dyspepsia, abnormal vision (including color vision changes), nasal congestion, back pain. Many lessen over the first week or two of treatment.
What should not be combined with Sildenafil?
The most clinically important interactions are with nitrates / nitric oxide donors (organic nitrates or nitrites, in any form): contraindicated - potentiates hypotensive effects and can cause fatal hypotension, alpha blockers: initiate viagra at 25 mg once the patient is stable on alpha-blocker therapy, antihypertensives: potentiates the hypotensive effect, ritonavir: do not exceed a maximum single dose of 25 mg in a 48 hour period (11-fold increase in auc). Always give a pharmacist or prescriber a complete list of everything you take, including over-the-counter drugs and supplements.
Sources & references
Important
This site provides educational information only and does not sell, prescribe or dispense medications. Always consult a licensed healthcare provider. This page is not medical advice.