What Is Sildenafil?
Sildenafil is a selective phosphodiesterase type 5 (PDE5) inhibitor that treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation.1 You take it as an oral tablet about an hour before sex.
Approved by the FDA in 1998 as Viagra, sildenafil was the first oral ED medication on the market. It's also sold as Revatio for pulmonary arterial hypertension. Generic versions became available in 2017, making it significantly more affordable than the brand-name product.
Sildenafil works on-demand — you don't take it daily like testosterone therapy. 59-84% of men report improved erections at standard doses.1 Many TRT clinics prescribe it during the first 6-12 weeks of testosterone replacement, when libido may return before erectile function fully recovers.
It requires sexual stimulation to work. Without arousal, sildenafil won't trigger an erection — it enhances your body's natural response to sexual cues rather than creating one artificially.
Sildenafil at a Glance
Sildenafil is a short-acting PDE5 inhibitor that enhances erections by increasing penile blood flow during sexual stimulation.
It's best for men who want on-demand ED treatment with precise timing and don't mind planning an hour ahead.
- Mechanism: Blocks PDE5 to prolong blood vessel dilation in the penis
- Dosing: 25-100 mg orally, 30-60 min before sex, max once daily
- Safety: Avoid with nitrates; watch for headache, flushing, visual changes
- Cost: $2-10/dose generic, $20-70/month typical use
How Sildenafil Works
Here's the practical takeaway: sildenafil keeps your penile blood vessels relaxed longer during arousal. The clinical detail — it blocks an enzyme called PDE5 that normally breaks down cyclic GMP, a chemical messenger that dilates arteries in the penis.2
During sexual stimulation, your body releases nitric oxide in penile tissue. This activates an enzyme that produces cyclic GMP, which relaxes smooth muscle in arterial walls and increases blood flow. PDE5 naturally degrades cyclic GMP after a few minutes. Sildenafil competitively binds to PDE5's active site — because its structure mimics cyclic GMP — and prevents breakdown, prolonging the vasodilation effect.3
You'll feel the effect within 30-60 minutes of taking a dose. Peak blood levels hit around an hour post-dose, with effectiveness lasting 4-5 hours.1 That's your functional window — shorter than tadalafil's 36-hour duration but long enough for most sexual encounters.
Absorption and Duration
Sildenafil has roughly 40% bioavailability after oral administration due to first-pass liver metabolism.1 Taking it with a high-fat meal can delay absorption by 30-60 minutes — not a major issue if you plan ahead, but worth noting if timing matters.
The half-life is 3-5 hours for the parent drug and 4 hours for its active metabolite (which has 50% of sildenafil's potency).1 Your liver clears it primarily through CYP3A4 metabolism, with about 80% excreted in feces and 13% in urine. If you take medications that inhibit CYP3A4 — like certain antifungals or HIV protease inhibitors — your sildenafil levels can spike.
How It Compares to Alternatives
Sildenafil is shorter-acting than tadalafil (Cialis), which lasts up to 36 hours and allows daily low-dose use. That makes tadalafil better for spontaneity, while sildenafil works better if you want precise timing and don't mind planning an hour ahead.
Head-to-head trials show similar efficacy — 71% success rate for sildenafil vs. 74% for tadalafil in completing intercourse attempts.2 The choice usually comes down to lifestyle fit. Sildenafil also has 10-fold higher selectivity for PDE5 over PDE6 (an enzyme in the retina), which explains why some men report temporary blue-tinted vision at higher doses.
Cyclic GMP is a chemical messenger molecule that relaxes smooth muscle in artery walls, allowing blood vessels to dilate and increasing blood flow to penile tissue during sexual stimulation.
PDE5 (phosphodiesterase-5) is an enzyme that naturally breaks down cyclic GMP, terminating vasodilation; sildenafil blocks this enzyme to prolong the blood-flow-enhancing effect.
Dosing & Administration
Standard starting dose is 50 mg taken about an hour before sexual activity. You can adjust to 25 mg if you get side effects or bump to 100 mg if 50 mg doesn't work.1 Maximum one dose per 24 hours.
Take it on an empty stomach or after a light meal for fastest absorption. A heavy, high-fat meal can delay onset by 30-60 minutes. Swallow the tablet whole with water — no need to time it with food otherwise.
| Parameter | Standard Protocol | Clinical Notes |
|---|---|---|
| Starting Dose | 50 mg | Take ~1 hour before sexual activity |
| Dose Range | 25-100 mg | Titrate based on response and tolerability |
| Frequency | As needed, max once daily | Not for continuous daily use (unless Revatio for PAH) |
| Timing | 30-60 min pre-activity | Effective window lasts 4-5 hours |
| Food Impact | Preferably without heavy meals | High-fat food delays absorption 30-60 min |
| Monitoring | None required for ED use | Check baseline liver/kidney function if impaired |
Adjustments for Special Cases
If you're over 65, start at 25 mg — older men clear the drug more slowly and get higher blood levels.1 Same adjustment if you have mild-to-moderate liver disease (max 50 mg). Severe liver impairment is a contraindication.
For kidney disease, use 25 mg if your creatinine clearance is below 30 mL/min, and max 50 mg if it's 30-50 mL/min.2 These dose caps prevent accumulation.
Side Effects & Safety
Most side effects are mild and dose-related. Headache and flushing are the most common — they happen because sildenafil causes mild systemic vasodilation, not just in the penis.1
The table below breaks down side effects by how often they occur. Serious adverse events are rare if you avoid contraindicated drug combinations.
| Frequency | Side Effect | Severity |
|---|---|---|
| Common (>10%) | Headache | Mild |
| Common (>10%) | Flushing | Mild |
| Occasional (1-10%) | Dyspepsia (indigestion) | Mild |
| Occasional (1-10%) | Nasal congestion | Mild |
| Occasional (1-10%) | Visual disturbances (blue tint, light sensitivity) | Mild-Moderate |
| Rare (<1%) | Priapism (erection >4 hours) | Severe (medical emergency) |
| Rare (<1%) | Sudden vision loss (NAION) | Severe |
| Rare (<1%) | Sudden hearing loss | Moderate-Severe |
When to Stop and Seek Help
If you get an erection lasting longer than 4 hours, go to the ER. Priapism can cause permanent tissue damage if untreated.
Sudden vision or hearing loss — even if temporary — means stop the drug and call your doctor immediately. These are rare but serious events linked to PDE5 inhibitors.
Cost & Access
Generic sildenafil costs $2-10 per 50 mg tablet at most pharmacies, making a typical month (4-8 doses) run $20-70. Brand-name Viagra is significantly more expensive — often $60-90 per pill without insurance.1
Insurance coverage varies. Many plans cover generic sildenafil with prior authorization showing you've tried other ED treatments or have a documented medical need. Some plans cap the monthly quantity (e.g., 6-12 tablets per month). Medicare Part D typically doesn't cover ED drugs, but some standalone plans do.
Where to Get It
You'll need a prescription from a physician, urologist, or men's health clinic. Telehealth platforms like Hims, Roman, and Rex MD offer online consultations and ship generic sildenafil directly — usually $20-40/month for 4-8 doses depending on subscription plans.
Compounded sildenafil is available through specialty pharmacies, often in custom strengths (e.g., 30 mg, 60 mg) or combined formulations (sildenafil + tadalafil). Pricing is similar to generic, sometimes slightly lower. Compounding makes sense if standard doses don't fit your needs.
Local TRT clinics often bundle sildenafil into their treatment packages. Expect to pay $30-60/month as part of a subscription that includes testosterone therapy and other ancillaries.