Sildenafil (Generic) vs Vardenafil (Levitra/Staxyn): Switching Between Them

What Are Sildenafil and Vardenafil, and How Do They Work?

Both sildenafil and vardenafil block phosphodiesterase type 5 (PDE5), the enzyme that degrades cyclic GMP in smooth muscle cells of the corpus cavernosum. Blocking PDE5 allows cyclic GMP to accumulate, smooth muscle to relax, and arterial inflow to increase in response to sexual stimulation. Neither drug produces an erection without arousal.

The PDE5 Inhibitor Mechanism

The landmark Goldstein et al. Trial published in the New England Journal of Medicine in 1998 (N=532) demonstrated that sildenafil produced significantly improved erections versus placebo at doses of 25, 50, and 100 mg, with 69% of attempts resulting in successful intercourse at the 100 mg dose compared with 22% on placebo 1. That trial established the PDE5 inhibitor class as the first-line oral therapy for erectile dysfunction and set the benchmark every later drug has been measured against.

Vardenafil entered the market roughly five years after sildenafil. Its IC50 for PDE5 is approximately 0.7 nM, compared with sildenafil's IC50 of roughly 3.9 nM, meaning vardenafil binds PDE5 with greater potency on a per-milligram basis. Whether that translates into a clinically meaningful difference for most men is debated.

PDE6 Selectivity and Visual Side Effects

Sildenafil also inhibits PDE6, the enzyme responsible for phototransduction in retinal rods and cones. That cross-reactivity explains the blue-tinge visual disturbance (cyanopsia) that roughly 3% of sildenafil users report at the 100 mg dose. Vardenafil is estimated to have roughly 10-fold greater selectivity for PDE5 over PDE6 compared with sildenafil, which may explain its lower rate of color-vision side effects in clinical trials 2.

Dosing: Sildenafil 20 to 100 mg vs Vardenafil 5 to 20 mg

Dose ranges differ considerably between the two drugs, and the difference matters when switching.

Sildenafil Dosing Details

The FDA-approved starting dose for erectile dysfunction is 50 mg taken 30 to 60 minutes before sexual activity, with a permitted range of 25 to 100 mg once daily 3. Lower 20 mg tablets are commercially available as Revatio (the pulmonary arterial hypertension indication) and are often prescribed off-label for ED at lower cost, with men taking two or three tablets to approximate the 50 mg or 100 mg dose.

Vardenafil Dosing Details

The recommended starting dose of vardenafil (Levitra) is 10 mg taken 60 minutes before activity, with dose adjustment to 5 mg or 20 mg based on response and tolerability 4. Staxyn, the orally disintegrating formulation, is fixed at 10 mg and should not be substituted on a milligram-for-milligram basis with Levitra tablets because bioavailability differs.

Men with hepatic impairment (Child-Pugh B) should start vardenafil at 5 mg, and the maximum is 10 mg in that population. A comparable dose reduction applies to sildenafil in severe hepatic impairment.

Efficacy Comparison: What the Data Show

No large, well-powered head-to-head randomized controlled trial has directly compared sildenafil versus vardenafil in a single study powered for superiority. The evidence base consists of individual trials and indirect comparisons.

Sildenafil Efficacy Data

Goldstein et al. (N=532, NEJM 1998) reported a mean IIEF (International Index of Erectile Function) erectile function domain score of 17.0 for sildenafil 100 mg versus 10.4 for placebo at study end, a difference of 6.6 points that exceeded the established minimal clinically important difference of 4 points 1. Successful intercourse rates were 57% on sildenafil 50 mg and 69% on sildenafil 100 mg versus 22% on placebo.

A 2005 Cochrane review of PDE5 inhibitors found that sildenafil improved the erectile function domain score by a mean of 6.9 points over placebo across pooled studies 5.

Vardenafil Efficacy Data

Porst et al. (Int J Impot Res, 2003, N=452) specifically examined vardenafil in men with erectile dysfunction and type 2 diabetes, a population that typically responds less robustly to PDE5 inhibitors. Vardenafil 10 mg and 20 mg produced statistically significant improvements in IIEF erectile function domain scores of 4.2 and 4.9 points respectively versus placebo (P<0.001 for both doses), with successful penetration rates of 52% and 56% on active drug versus 22% on placebo 2. The finding is notable because diabetic ED is widely considered one of the hardest-to-treat subtypes.

A 2006 meta-analysis by Tsertsvadze et al. Published in Annals of Internal Medicine found no statistically significant difference in IIEF erectile function domain scores between sildenafil, vardenafil, and tadalafil in indirect comparisons, and stated: "The three PDE-5 inhibitors had similar efficacy and safety profiles for treatment of erectile dysfunction" 6.

Diabetic and Difficult-to-Treat ED

For men with diabetic ED, both drugs work. Porst et al. Demonstrated vardenafil's benefit in this subgroup 2. Sildenafil has similarly been studied in diabetic populations; a trial by Rendell et al. (JAMA 1999, N=268) reported 56% of diabetic men on sildenafil had improved erections versus 10% on placebo 7. Choosing between the two for diabetic ED is therefore primarily a matter of individual response and tolerability rather than evidence-based superiority of one agent.

Side Effects: Sildenafil vs Vardenafil

The side-effect profiles overlap substantially because both drugs share the PDE5 mechanism. Differences are largely a matter of degree, not kind.

Shared Side Effects

Both drugs produce vasodilatory effects that can cause headache (10 to 16% of users), flushing (5 to 12%), nasal congestion (4 to 9%), and hypotension, particularly when combined with alpha-blockers or antihypertensives 3. Dyspepsia occurs with both, though less prominently than with tadalafil.

Where They Differ

Sildenafil at doses of 50 to 100 mg produces cyanopsia (transient blue-tinge vision) in approximately 3% of users due to PDE6 inhibition 1. Vardenafil's reduced PDE6 affinity means color-vision changes are reported less frequently. On the other hand, vardenafil has a small QTc-prolonging effect that sildenafil does not share, which is why the FDA label for vardenafil carries a warning against use in patients taking class Ia or III antiarrhythmics 4. Men with known QT prolongation, hypokalemia, or hypomagnesemia should avoid vardenafil.

Rare but Serious Risks

Both drugs carry a rare risk of non-arteritic anterior ischemic optic neuropathy (NAION), sudden hearing loss, and prolonged erection (priapism). Neither drug should be used within 24 hours of a nitrate dose; the combination causes severe hypotension and has resulted in deaths 8.

How to Switch from Sildenafil to Vardenafil (or Vice Versa)

Switching between sildenafil and vardenafil is straightforward because both drugs have similar half-lives of approximately 4 hours and neither accumulates meaningfully with once-daily sexual-occasion dosing. No pharmacokinetic washout period is required before taking the new drug.

Dose-Equivalence Guidance

A practical starting-point conversion used in clinical practice is:

• Sildenafil 25 mg corresponds roughly to vardenafil 5 mg
• Sildenafil 50 mg corresponds roughly to vardenafil 10 mg
• Sildenafil 100 mg corresponds roughly to vardenafil 20 mg

These are approximations based on the relative PDE5 IC50 values and approved dose ranges, not pharmacokinetic equivalence studies. Individual response varies, and patients should titrate after switching under physician guidance. The American Urological Association guideline on erectile dysfunction states that "dose titration to optimize the balance between efficacy and side effects is recommended for all PDE5 inhibitors" 9.

Step-by-Step Switch Protocol

1. Take the last sildenafil dose as usual.
2. At the next sexual occasion (minimum 4 hours after the last sildenafil dose, consistent with label half-life), start vardenafil at the dose-equivalent starting point above.
3. Assess response and side effects over 4 to 6 attempts before adjusting dose.
4. If switching in the opposite direction (vardenafil to sildenafil), apply the same timing and equivalent-dose logic.

Men switching because sildenafil caused visual side effects may find vardenafil more comfortable at the equivalent dose. Men switching because vardenafil caused palpitations or who have borderline QTc intervals should confirm cardiac safety with their prescriber before starting vardenafil.

When Not to Switch Without Medical Review

Do not self-switch if you are taking any of the following: nitrates (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate), riociguat, alpha-blockers (tamsulosin, doxazosin) at doses higher than tamsulosin 0.4 mg, class Ia antiarrhythmics (quinidine, procainamide), or class III antiarrhythmics (amiodarone, sotalol). The QTc risk of vardenafil makes the antiarrhythmic interactions drug-specific, not class-wide.

Cost and Availability

Sildenafil is less expensive in the United States. Generic sildenafil became available following Pfizer's patent expiry, and retail prices commonly fall between $1 and $10 per tablet depending on dose and pharmacy. GoodRx pricing data show generic sildenafil 100 mg available at some pharmacies for under $2 per tablet with a discount coupon.

Generic vardenafil (generic Levitra) is available in the United States but at higher prices than generic sildenafil, ranging from roughly $8 to $25 per tablet at retail without a coupon. Staxyn (orally disintegrating vardenafil) remains branded and commands a premium. For men primarily motivated by cost, sildenafil offers a clear advantage.

Drug Interactions Specific to Each Agent

Sildenafil-Specific Interactions

Sildenafil is metabolized primarily by CYP3A4 and partially by CYP2C9. Potent CYP3A4 inhibitors, including ritonavir, ketoconazole, and itraconazole, significantly increase sildenafil plasma levels. The FDA label caps sildenafil at 25 mg per 48 hours in patients on ritonavir 3.

Grapefruit juice (500 mL or more) inhibits intestinal CYP3A4 and may increase sildenafil exposure by 23 to 47%, a clinically modest but real interaction 10.

Vardenafil-Specific Interactions

Vardenafil is also CYP3A4-metabolized, so the same inhibitor warnings apply. Ketoconazole 200 mg increases vardenafil AUC by roughly 10-fold; the label recommends a maximum of 5 mg per 24 hours in that context 4. The QTc interaction with antiarrhythmics is unique to vardenafil and has no sildenafil equivalent.

Who Should Choose Sildenafil and Who Should Choose Vardenafil?

The answer depends on individual medical history, not on which drug is broadly "better."

Sildenafil May Be Preferable When:

• Cost is a primary concern (generic widely available and cheap)
• The man has no history of troublesome visual side effects at his usual dose
• The prescriber wants the drug with the longest clinical track record (approved by FDA in March 1998) 11
• Cardiac rhythm issues (QTc prolongation, arrhythmia history) make vardenafil's QTc effect a concern

Vardenafil May Be Preferable When:

• The man experiences bothersome cyanopsia on sildenafil 50 to 100 mg
• He prefers an orally disintegrating tablet (Staxyn) for convenience or discretion
• His prescriber judges that higher PDE5 selectivity is worth the cost premium
• He has not responded adequately to sildenafil at maximum dose and wants to try a different molecule before escalating to tadalafil

Neither drug is categorically superior. The Tsertsvadze et al. Meta-analysis in Annals of Internal Medicine found no significant efficacy difference among approved PDE5 inhibitors 6. Individualization is the standard of care.

Food, Alcohol, and Lifestyle Factors

High-fat meals delay sildenafil absorption and reduce peak plasma concentration (Cmax) by 29% and extend time to Cmax by 60 minutes 3. Men who take sildenafil and find it inconsistent should consider whether meal timing is a factor.

Vardenafil shows a similar food-related delay, though the magnitude is slightly smaller. Staxyn (ODT formulation) should specifically not be taken with any liquid other than water, and food interactions have not been fully characterized for the ODT relative to the tablet.

Alcohol at moderate doses (up to two standard drinks) does not appear to significantly worsen sildenafil hypotension in otherwise healthy men, but higher alcohol intake potentiates vasodilation and increases fall risk. The same caution applies to vardenafil. Neither drug should be combined with poppers (amyl nitrite or butyl nitrite), which are organic nitrates.