Men's Sexual Health
Sildenafil (Viagra): How It Works, Dosing, and Safety
Medically reviewed by HealthRX.com Medical Team · Last reviewed
What is sildenafil and how does it work?
Sildenafil is the original PDE5 inhibitor, first sold as Viagra. During arousal it increases blood flow to the penis by blocking the enzyme that breaks down the cGMP signal. It works with sexual stimulation, not on its own.
Peak levels are reached in about 30 to 120 minutes, with a median near 60 minutes, which is why it is usually taken about an hour before sex. [4]
How is sildenafil dosed and timed?
The usual starting dose is 50 mg taken about an hour before sex, adjustable between 25 and 100 mg, with a maximum of once per day. It is an on-demand medicine only; there is no daily-dose ED regimen for sildenafil. A high-fat meal can slow and blunt its onset.
| Feature | Detail | Notes |
|---|---|---|
| Timing | About 1 hour before | Window of 30 minutes to 4 hours |
| Dose | 25, 50 or 100 mg | Maximum once per day |
| Food | High-fat meal delays onset | Peak delayed about 60 minutes |
Is sildenafil safe, and what are the side effects?
Sildenafil is generally safe for most men but must never be combined with nitrates, which can cause a dangerous drop in blood pressure. Common side effects include headache, flushing, indigestion and nasal congestion. A temporary blue tinge or altered color vision can occur and is specific to sildenafil.
- Never take with nitrates or poppers; this is an absolute contraindication. [4]
- Use caution with alpha-blockers; a lower 25 mg start may be advised. [4]
- Common: headache, flushing, indigestion, nasal congestion, mild visual changes.
- Seek urgent care for sudden vision loss, an erection over four hours, or chest pain. [4]
Why can sildenafil cause blue-tinted vision?
Sildenafil weakly affects PDE6 in the retina, which can cause a temporary, mild blue tinge to vision, light sensitivity or altered color vision. This is harmless and passes as the drug wears off. It is different from sudden vision loss, which is rare and needs urgent care.
This visual effect is more characteristic of sildenafil than tadalafil, and is one reason some men prefer one over the other. [6]
How does Sildenafil compare with other peptides?
| Sildenafil | Tadalafil | |
|---|---|---|
| Onset | About 30 to 60 minutes | Peak around 2 hours |
| Duration | Several hours | Up to 36 hours |
| Dosing | On demand only | On demand or daily |
| Signature effect | Mild visual changes | Back or muscle ache |
Frequently asked questions
How long before sex should I take sildenafil?
About an hour before, though the label allows anywhere from 30 minutes to 4 hours before activity. It usually works within 30 to 60 minutes and lasts several hours. It needs sexual stimulation to work.
Does food affect sildenafil?
Yes. A high-fat meal can slow and blunt onset, delaying the peak by around an hour and lowering the peak level. Taking it on a lighter stomach can help it work more predictably.
Why does sildenafil sometimes cause blue vision?
Sildenafil weakly affects an enzyme in the retina, which can cause a temporary, harmless blue tinge or altered color vision. It passes as the drug wears off. Sudden vision loss is different and rare, and needs urgent care.
Can I take sildenafil with my heart medications?
Never combine sildenafil with nitrate heart medicines such as nitroglycerin, or with poppers; this can cause a dangerous drop in blood pressure. Alpha-blockers also need caution. Share every medication with your prescriber.
Is sildenafil or tadalafil better?
Neither is universally better. Sildenafil is shorter-acting and food can delay it. Tadalafil lasts up to 36 hours and offers a daily option. The best choice depends on your health, other medicines and how spontaneous you want to be.
Citations
- VIAGRA (sildenafil citrate) tablets, FDA Prescribing Information (DailyMed).
- Corbin JD. Mechanisms of action of PDE5 inhibition in erectile dysfunction. Int J Impot Res. 2004;16(Suppl 1):S4-S7.
- Dhaliwal A, Gupta M. PDE5 Inhibitors. StatPearls. NCBI Bookshelf; updated 2023.
This guide is educational and is not a substitute for individualized medical advice. Sildenafil is prescription-only and requires evaluation by a licensed provider.