Cialis vs Viagra vs Levitra vs Stendra: Complete ED Medication Comparison

PDE5 inhibitors — sildenafil (Viagra), tadalafil (Cialis), vardenafil (Levitra), and avanafil (Stendra) — are the most prescribed and most studied class of drugs in men's sexual health. All four work through the same core mechanism, but they differ meaningfully in how quickly they work, how long they last, what foods and drugs interact with them, and which side effects are most common. Understanding those differences helps men and their physicians select the right medication rather than simply defaulting to the most familiar name.

How PDE5 Inhibitors Work

Erection requires increased blood flow into the corpora cavernosa — the two cylindrical chambers of the penis. This process begins with sexual stimulation, which triggers nitric oxide (NO) release from penile nerve endings and endothelial cells. NO activates guanylyl cyclase, which produces cyclic GMP (cGMP). cGMP is the key signaling molecule that relaxes smooth muscle in penile arterial walls, causing vasodilation and increased blood inflow that produces an erection.

PDE5 (phosphodiesterase type 5) is an enzyme that breaks down cGMP. In men with ED, inadequate cGMP signaling — due to vascular disease, neurological issues, hormonal problems, or other factors — means the smooth muscle relaxation response is insufficient. PDE5 inhibitors block PDE5's activity, preventing cGMP degradation and allowing it to accumulate to levels sufficient to trigger vasodilation.

The critical point: PDE5 inhibitors amplify an existing signal — they do not create an erection from scratch. Sexual stimulation is still required. A man who takes a PDE5 inhibitor and is not sexually aroused will not develop an erection. This is why PDE5 inhibitors are ineffective for men with low libido or primarily psychogenic ED without a vasodilatory component.

PDE5 inhibitors are also absolutely contraindicated with nitrate medications (nitroglycerin, isosorbide mono/dinitrate) — both reduce blood pressure through separate mechanisms, and their combination can cause severe, potentially fatal hypotension. Any man taking nitrates for chest pain, heart disease, or pulmonary hypertension must inform their prescribing physician before taking any PDE5 inhibitor.

Comparison Table: Onset, Duration, Dosing

A side-by-side overview of the four major PDE5 inhibitors:

• Sildenafil (Viagra): Onset 30–60 min | Duration 4–6 hours | Standard dose 50–100 mg | Food interaction: High-fat meal delays absorption by 1 hour | Approved 1998
• Tadalafil (Cialis) as-needed: Onset 30–45 min | Duration up to 36 hours | Standard dose 10–20 mg | Food: No significant interaction | Approved 2003
• Tadalafil (Cialis) daily: Continuous effect | Standard dose 2.5–5 mg | Same no-food-interaction advantage | Requires consistent daily dosing
• Vardenafil (Levitra): Onset 25–60 min | Duration 4–5 hours | Standard dose 10–20 mg | Food: High-fat meal delays absorption | Approved 2003
• Avanafil (Stendra): Onset 15–30 min | Duration 6 hours | Standard dose 100–200 mg | Food: Minimal interaction | Approved 2012

The most practically important differences are duration (Cialis stands far apart at 36 hours vs 4–6 hours for the others) and onset (Stendra is fastest, often effective in 15 minutes).

Viagra (Sildenafil): What You Need to Know

Sildenafil was the first PDE5 inhibitor, discovered accidentally during Pfizer's development of a cardiovascular drug in the 1990s and approved by the FDA in 1998. Its brand name "Viagra" became one of the most recognized pharmaceutical names worldwide. Sildenafil's patent expired in 2017, and generic sildenafil is now available at a fraction of the brand-name cost.

Dosing: The starting dose is typically 50 mg, taken 30–60 minutes before anticipated sexual activity. This can be increased to 100 mg if the 50 mg dose is inadequate, or reduced to 25 mg in men with hepatic impairment, significant renal impairment, or those taking certain drugs that increase sildenafil levels (notably strong CYP3A4 inhibitors like ketoconazole or ritonavir). Maximum one dose per 24-hour period.

Food interaction: This is sildenafil's most clinically relevant practical limitation. A high-fat meal can delay sildenafil absorption by 60+ minutes and reduce peak plasma concentration by approximately 29%. Men who take sildenafil after a large, fatty dinner may find it works poorly — an experience that can be attributed to the medication being "ineffective" when in fact the issue was timing and food co-administration. For best results, sildenafil should be taken on an empty stomach or at least 2 hours after a light meal.

Clinical efficacy: The pivotal clinical trials for sildenafil demonstrated successful intercourse in approximately 70% of men with ED of various etiologies, compared to approximately 22% with placebo. Efficacy is slightly lower in men with diabetes (approximately 57%), post-prostatectomy (approximately 43%), and severe vasculogenic ED — reflecting the need for some intact NO pathway function.

Side effects: Headache (11%), flushing (10%), dyspepsia (4–7%), nasal congestion (4%), visual disturbances — most notably a transient blue-green color tinge (2–3%, related to mild PDE6 inhibition in the retina). Sildenafil has the most significant PDE6 cross-reactivity of the four agents, which explains the visual side effect unique to sildenafil at higher doses.

Cialis (Tadalafil): Daily and As-Needed Options

Tadalafil (Cialis) is distinguished by two features that make it unique in its class: its 36-hour duration of action and its availability in a daily low-dose formulation. The 36-hour window earned it the nickname "the weekend pill" — a single dose on Friday evening provides coverage through Sunday without additional dosing.

As-needed dosing (Cialis 10 or 20 mg): Onset is 30–45 minutes, with peak concentrations at approximately 2 hours. Duration extends to 36 hours in most men. Food does not significantly affect tadalafil absorption — a major practical advantage over sildenafil. Men can take tadalafil with a large meal without concern about delayed onset. Starting dose is 10 mg; increase to 20 mg if needed.

Daily dosing (Cialis 2.5 or 5 mg): Daily low-dose tadalafil maintains steady-state plasma concentrations, providing continuous PDE5 inhibition. This allows for spontaneous sexual activity without planning a pill around an expected event — an important quality-of-life difference for many men. Daily Cialis also has FDA approval for benign prostatic hyperplasia (BPH), making it relevant for older men with both ED and urinary symptoms. Response to daily Cialis is assessed after 4 weeks at 5 mg; if response is inadequate, the as-needed 20 mg approach may be tried.

Side effects: Similar to sildenafil — headache, flushing, dyspepsia. A notable tadalafil-specific side effect is back pain and myalgia (muscle aches), reported in approximately 5–6% of men. These appear to be related to PDE11 inhibition — tadalafil has slightly more PDE11 cross-reactivity than the other agents, and PDE11 is expressed in skeletal muscle. Back pain typically appears 12–24 hours after dosing and resolves within 48 hours. It is not harmful but is uncomfortable and leads some men to prefer a different agent.

Levitra (Vardenafil) and Stendra (Avanafil)

Vardenafil (Levitra): Approved in 2003, vardenafil shares a similar pharmacokinetic profile to sildenafil — onset 25–60 minutes, duration 4–5 hours — but has slightly higher potency (lower IC50 for PDE5) at equivalent milligram doses. It also shares sildenafil's food interaction limitation. Vardenafil has some data suggesting superior efficacy in men with diabetes compared to sildenafil, though head-to-head evidence is limited.

An orally disintegrating tablet form of vardenafil (Staxyn) dissolves on the tongue and bypasses the food interaction issue. It is not bioequivalent to the standard tablet (it has higher bioavailability) and should not be substituted on a milligram-for-milligram basis without physician guidance.

Vardenafil has one important safety consideration: it can prolong the cardiac QT interval at higher doses. Men with known QT prolongation or who are taking other QT-prolonging medications should have a specific cardiology consultation before using vardenafil.

Avanafil (Stendra): The newest of the four, approved in 2012, avanafil was designed for selectivity — it has the highest PDE5 selectivity ratio among the four agents, meaning it inhibits PDE5 much more potently relative to other PDE isoforms. This selectivity profile results in fewer off-target side effects: avanafil has the lowest rate of visual side effects (near zero) and significantly less flushing and back pain than the other agents.

Avanafil's most distinctive clinical advantage is onset — it can be effective in as little as 15 minutes in some men, compared to 30–60 minutes for sildenafil and vardenafil. A 2012 phase 3 study showed avanafil 200 mg produced successful intercourse in 47% of men when taken as little as 15 minutes before activity. This rapid onset is particularly valued by men who want a more spontaneous experience without the longer planning window required by sildenafil.

The primary drawback of avanafil: it lacks generic availability as of 2026 and is considerably more expensive than generic sildenafil or tadalafil.

Side Effects and Drug Interactions

All four PDE5 inhibitors share a common side effect profile reflecting their vasodilatory mechanism:

• Headache: Most common — reported in 10–20% of men across all agents. Caused by cerebral vasodilation. Typically mild and resolves within a few hours.
• Flushing: Facial redness and warmth due to skin vasodilation. Common (10–15%) with sildenafil and vardenafil; less common with avanafil.
• Nasal congestion: Vasodilation of nasal mucosa. Affects approximately 4–9% of users.
• Dyspepsia (heartburn/indigestion): Reported by 4–12% of men, particularly with sildenafil and vardenafil.
• Visual disturbances: Blue-green tinge, blurred vision — most common with sildenafil (2–3%), very rare with avanafil.
• Hypotension: All PDE5 inhibitors lower blood pressure modestly. Additive hypotension with alpha-blockers (like tamsulosin for BPH) is a clinically important interaction — use with caution and typically start at lowest dose.

Key drug interactions to know:

• Nitrates (absolutely contraindicated): Severe hypotension. No PDE5 inhibitor should ever be taken with organic nitrates.
• Alpha-blockers: Can cause additive hypotension. Start PDE5 inhibitor at lowest dose and allow stabilization of alpha-blocker first. Tadalafil has the most data for combined use with alpha-blockers.
• CYP3A4 inhibitors (ketoconazole, ritonavir, clarithromycin, grapefruit juice): Increase PDE5 inhibitor plasma levels. Dose reduction may be required.
• CYP3A4 inducers (rifampin, carbamazepine): Decrease PDE5 inhibitor levels and may reduce efficacy.

Generic vs Brand-Name: Cost and Quality

The patent cliff for sildenafil and tadalafil has dramatically reduced cost for both medications:

• Generic sildenafil: Available since 2017. Cost: $1–$4 per tablet at major pharmacies with GoodRx or similar discount programs, compared to $50–$80 per Viagra tablet. Bioequivalent to Viagra — same active ingredient, same FDA-approved manufacturing standards.
• Generic tadalafil: Available since 2018. Cost: $2–$8 per tablet for as-needed doses; $1–$3/day for daily 5 mg. Bioequivalent to Cialis.
• Brand Levitra / generic vardenafil: Generics available; cost $3–$10 per tablet.
• Stendra (avanafil): No generic as of 2026. Brand-only cost: $30–$50 per tablet. This is the most expensive of the four and is less accessible for men paying out of pocket.

FDA-approved generic medications manufactured in licensed facilities are bioequivalent to their brand-name counterparts. The FDA requires generics to perform within 80–125% of the brand's pharmacokinetic parameters (a range that is clinically negligible). Choosing generic sildenafil or tadalafil over brand-name Viagra or Cialis is a financially sensible choice with no meaningful clinical sacrifice.

When Medications Aren't Enough

PDE5 inhibitors are highly effective for most men with vasculogenic ED, but approximately 30–35% of men do not achieve satisfactory response — the so-called "PDE5 inhibitor non-responders." This group includes men with severe vascular disease, complete penile denervation post-prostatectomy, or other conditions where cGMP signaling is fundamentally compromised rather than simply undermined.

For men who do not respond adequately to PDE5 inhibitors, the next steps include:

• Optimization of the PDE5 inhibitor trial: Many initial non-responders are actually responding to incorrect dosing, timing, food interaction, or insufficient sexual stimulation. A properly structured 8-session trial of maximum-dose sildenafil (100 mg) on an empty stomach with adequate arousal identifies more true responders before labeling someone a non-responder.
• Combination therapy: Adding TRT (if low testosterone is confirmed) to PDE5 inhibitor treatment significantly improves response rates in hypogonadal men.
• Intracavernosal injection (ICI) therapy: Direct injection of vasoactive agents (alprostadil alone, or combinations like Trimix — alprostadil + phentolamine + papaverine) into the corpora produces reliable erections in 85–90% of men including PDE5 non-responders. It requires self-injection technique training but is highly effective.
• Vacuum erection device (VED): Creates erections through external negative pressure. Effective but less spontaneous than medications. Often used in post-prostatectomy penile rehabilitation.
• Low-intensity shockwave therapy (GainsWave/Li-ESWT): May restore PDE5 inhibitor responsiveness in some vasculogenic non-responders by improving penile vascular health.
• Penile implant: The definitive treatment for men with medically refractory ED who have exhausted all other options. Delivers reliable, on-demand erections with satisfaction rates above 90%.