Sildenafil Overview, Dosing & Safety | Peptide Database

Sildenafil (Viagra)

PDE5 Inhibitor | Erectile Function & Pulmonary Hypertension

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Sildenafil is the first PDE5 inhibitor to reach the market and remains one of the most widely recognized pharmaceuticals in the world. Originally developed by Pfizer scientists investigating treatments for angina and hypertension, sildenafil's pronounced effect on erectile function was discovered during Phase I clinical trials in 1992. It received FDA approval for erectile dysfunction in 1998 under the brand name Viagra, and subsequently gained a second FDA approval in 2005 as Revatio for the treatment of pulmonary arterial hypertension (PAH). Sildenafil is a shorter-acting PDE5 inhibitor with a half-life of 3-5 hours and a therapeutic window of approximately 4-6 hours. Unlike tadalafil, sildenafil absorption is significantly reduced by high-fat meals, and its lower selectivity for PDE5 over PDE6 (the retinal phosphodiesterase) can produce transient visual disturbances such as a blue-tinged color shift at higher doses. Despite the availability of longer-acting alternatives, sildenafil remains a first-line treatment for ED due to its well-established efficacy, extensive safety data spanning over 25 years, and wide generic availability.

Sildenafil selectively inhibits phosphodiesterase type 5 (PDE5), the enzyme responsible for the hydrolysis and degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle. During sexual arousal, nitric oxide (NO) is released from nerve terminals and endothelial cells in the corpus cavernosum, activating soluble guanylate cyclase and increasing intracellular cGMP concentrations. Elevated cGMP activates protein kinase G (PKG), which phosphorylates multiple targets leading to smooth muscle relaxation, vasodilation of penile arteries, and engorgement of the corpora cavernosa with blood. By inhibiting PDE5, sildenafil prevents cGMP breakdown, prolonging and amplifying this vasodilatory cascade. Sildenafil requires sexual stimulation to be effective, as NO release is the initiating signal. In pulmonary arterial hypertension, the same PDE5 inhibition relaxes pulmonary vascular smooth muscle, reducing pulmonary artery pressure and pulmonary vascular resistance, thereby decreasing right ventricular afterload and improving exercise capacity. Sildenafil also has moderate inhibitory activity against PDE6, the phosphodiesterase isoform in retinal photoreceptors, which underlies the transient visual color changes reported by some users.

Molecular Data

Research Indications

First-line pharmacotherapy for ED of various etiologies. Demonstrated efficacy across a broad range of populations including patients with diabetes, spinal cord injury, post-radical prostatectomy, and psychogenic ED. Effective in approximately 70-85% of men with ED.

Rapid onset and predictable duration make sildenafil well-suited for planned sexual activity. The shorter therapeutic window provides a defined period of enhanced erectile response without prolonged pharmacological effect.

FDA-approved as Revatio (20mg TID) for PAH (WHO Group 1) to improve exercise capacity and delay clinical worsening. Reduces mean pulmonary artery pressure, pulmonary vascular resistance, and improves 6-minute walk distance.

Evidence from controlled trials demonstrates that sildenafil reduces the frequency and severity of Raynaud's attacks by enhancing digital blood flow through NO-cGMP pathway augmentation. Used off-label in refractory cases.

Acute and chronic PDE5 inhibition with sildenafil improves flow-mediated dilation and endothelial-dependent vasodilation across various cardiovascular risk populations.

Sildenafil has been studied for the prevention and treatment of high-altitude pulmonary edema (HAPE) by reducing exaggerated hypoxic pulmonary vasoconstriction. Used off-label by climbers and military personnel at extreme altitudes.

Used off-label in neonatal ICUs for persistent pulmonary hypertension of the newborn (PPHN) when inhaled nitric oxide is unavailable or insufficient. Evidence supports its use as an adjunct therapy.

Enhanced vasodilation may increase blood flow to working muscles. Some evidence suggests improved exercise performance at altitude via reduced pulmonary artery pressure. Effects at sea level are less well-established.

Dosing Protocols

Sildenafil is administered orally as film-coated tablets (Viagra: 25, 50, 100mg) or as an oral suspension (Revatio: 10mg/mL). For ED, it should be taken approximately 30-60 minutes before sexual activity. High-fat meals significantly delay absorption and reduce peak plasma concentration by approximately 29%, so taking sildenafil on an empty stomach or after a light meal is recommended for optimal effect.

ED (starting dose)

50mg

As needed, 30-60 min before sexual activity

Oral

ED (low dose)

25mg

As needed, for patients >65 years, hepatic impairment, renal impairment, or CYP3A4 inhibitor use

ED (maximum dose)

100mg

As needed, max once per 24 hours

Pulmonary arterial hypertension (Revatio)

20mg

Three times daily, approximately 4-6 hours apart

Interactions

What to Expect

Side Effects & Safety

Common Side Effects

Headache (16% incidence, most common side effect)

Flushing / warmth (10%, due to systemic vasodilation)

Dyspepsia / indigestion (7%)

Nasal congestion / rhinitis (4%)

Visual disturbances -- blue-tinged vision, increased brightness perception, blurred vision (3%, due to PDE6 cross-reactivity in retinal photoreceptors)

Dizziness (2%)

Diarrhea (3%)

Rash (2%)

Stop Signs - Discontinue if:

Priapism: erection lasting more than 4 hours -- seek emergency medical attention immediately to prevent ischemic damage and permanent erectile tissue injury

Sudden loss or decrease of vision in one or both eyes (possible NAION) -- discontinue and seek immediate ophthalmological evaluation

Sudden decrease or loss of hearing, with or without tinnitus and dizziness -- discontinue and seek immediate medical evaluation

Chest pain, angina, or symptoms of myocardial ischemia during or after sexual activity -- do not take nitrates, call emergency services

Severe hypotension: fainting, prolonged dizziness, or lightheadedness -- lie down and seek medical attention

Severe allergic reaction: difficulty breathing, throat swelling, widespread rash

Contraindications

Concurrent use of any organic nitrate medication (absolute contraindication)

Concurrent use of guanylate cyclase stimulators (e.g., riociguat)

Known hypersensitivity to sildenafil or any tablet excipient

Severe hepatic impairment (starting dose 25mg recommended in moderate impairment)

Severe renal impairment with CrCl <30 mL/min (starting dose 25mg recommended)

Recent stroke or myocardial infarction (within 6 months per original labeling)

Unstable angina or angina during sexual intercourse

Uncontrolled hypertension or symptomatic hypotension (resting BP <90/50 mmHg)

Hereditary degenerative retinal disorders including retinitis pigmentosa

History of NAION (relative contraindication -- increased recurrence risk)

Conditions predisposing to priapism (sickle cell anemia, multiple myeloma, leukemia)

Quality Checklist

Good Signs

Film-coated tablets with consistent shape, size, and color (branded Viagra: blue, diamond-shaped with 'Pfizer' deboss and 'VGR' + dose strength)

Clear labeling with manufacturer, lot number, expiration date, and NDC

Blister packaging or sealed bottle with tamper-evident features

Purchased from a licensed pharmacy or verified online pharmacy (VIPPS-accredited)

Generic versions manufactured by reputable pharmaceutical companies with FDA ANDA approval

Warning Signs

Unusually low pricing compared to established generic pricing

Tablets that vary in size, color, or markings within the same batch

Packaging with spelling errors, poor print quality, or missing regulatory information

Sold without requiring a prescription in jurisdictions where one is legally required

Online pharmacies that do not require any medical history or evaluation

Bad Signs

Tablets that crumble, have an unusual odor, or are discolored

Products sourced from unregulated overseas pharmacies with no quality verification

'Herbal' or 'natural' supplements found to contain undeclared sildenafil (a common finding in FDA testing)

No expiration date, lot number, or manufacturer information on packaging

Counterfeit packaging -- Pfizer Viagra has specific security features including hologram on box

Frequently Asked Questions

Does sildenafil work if you're not sexually stimulated?

No—sildenafil requires sexual stimulation to work. It doesn't directly cause erections but amplifies the natural erectile response to arousal. Without sexual activation or fantasy, sildenafil alone won't produce erections, making it useless for non-responsive penises or when there's no arousal stimulus.

Is sildenafil safer than tadalafil (Cialis) for daily use?

Sildenafil's shorter 3-5 hour window makes it better for as-needed dosing than daily use, while tadalafil's 36-hour window is better for daily low-dose regimens. Daily sildenafil requires multiple daily doses to maintain coverage, whereas daily tadalafil (2.5-5mg) provides continuous effect—different safety profiles, not one universally safer.

Why does sildenafil cause blue vision but tadalafil doesn't?

Sildenafil has weaker selectivity for PDE5 over PDE6 (the retinal isoform), causing the characteristic blue-tinged vision at higher doses. Tadalafil's structure provides much higher PDE5/PDE6 selectivity, making visual disturbances rare. This is why sildenafil is dosed as needed (peak effect) while tadalafil suits continuous dosing.

Can I combine sildenafil with poppers (amyl nitrite)?

Absolutely not—this combination causes potentially fatal hypotension (severe drop in blood pressure). The combination is dangerous enough that it's listed as an absolute contraindication. Any nitrate (including recreational poppers) is contraindicated with PDE5 inhibitors and can cause syncope, stroke, or cardiac events.

References

The SUPER-1 trial (Sildenafil Use in Pulmonary Arterial Hypertension) demonstrated that sildenafil 20mg, 40mg, and 80mg TID all significantly improved 6-minute walk distance, WHO functional class, and mean pulmonary artery pressure compared to placebo in 278 PAH patients. This pivotal trial led to FDA approval of sildenafil (Revatio) for PAH.

Landmark pivotal trial establishing sildenafil efficacy for ED. In 532 men with organic, psychogenic, or mixed ED, sildenafil 25-100mg significantly improved erections in 69% of all attempts vs 22% with placebo. Demonstrated dose-dependent improvements in IIEF scores and intercourse success across all ED etiologies.

One of the earliest published clinical studies of sildenafil for ED. Demonstrated significant dose-dependent improvements in erectile function in men with mild to moderate ED in a double-blind, placebo-controlled crossover study. Established the therapeutic dose range of 25-100mg and confirmed the requirement for sexual stimulation.

Demonstrated that sildenafil 100mg produced modest hemodynamic effects in patients with severe coronary artery disease during cardiac catheterization, including a 7% reduction in mean arterial pressure and 13% reduction in systemic vascular resistance. Confirmed the safety of sildenafil in cardiac patients when nitrates are not co-administered.

Comprehensive pharmacological comparison of PDE5 inhibitors. Sildenafil demonstrated a Tmax of approximately 1 hour, half-life of 3-5 hours, and significant food interaction with high-fat meals. PDE5/PDE6 selectivity ratio of approximately 7:1 (vs >780:1 for tadalafil) accounts for visual disturbance profile unique to sildenafil.

Related Peptides

No known pharmacological interaction. BPC-157's effects on angiogenesis and NO system may theoretically complement sildenafil, though clinical data on the combination is lacking.

Disclaimer

This information is for educational and research purposes only. Consult a healthcare professional before use.