Viagra vs Vardenafil (Levitra/Staxyn): Switching Between Them

How Sildenafil and Vardenafil Work

Both drugs block phosphodiesterase type 5, the enzyme that degrades cyclic GMP in penile smooth muscle. When PDE5 is inhibited, cGMP accumulates, smooth muscle relaxes, and blood flow into the corpora cavernosa increases. Neither drug creates an erection on its own. Sexual stimulation is still required to trigger nitric oxide release and initiate the cGMP cascade 1.

The molecular difference matters at the selectivity level. Sildenafil inhibits PDE5 with an IC50 of approximately 3.5 nM but also inhibits PDE6 (the retinal phosphodiesterase) at an IC50 of roughly 34 nM, giving a selectivity ratio of about 10:1 2. Vardenafil's IC50 for PDE5 is approximately 0.7 nM, with a PDE6 IC50 near 11 nM, yielding a selectivity ratio closer to 15:1 3. That higher PDE5 potency is why vardenafil's effective dose (10-20 mg) is a fraction of sildenafil's (50-100 mg). The clinical implication of the selectivity gap is most visible in one specific side effect: blue-tinted vision (cyanopsia), reported in 3-11% of men on sildenafil but rarely with vardenafil 4.

Both agents share the same contraindication profile. Concurrent nitrate use is absolutely contraindicated. Alpha-blocker co-administration requires dose separation and hemodynamic caution.

Efficacy Comparison: What the Trials Show

No single large randomized trial has directly compared sildenafil head-to-head against vardenafil with a primary erectile-function endpoint. The evidence base relies on separate placebo-controlled registration trials and a handful of crossover or preference studies.

Sildenafil's landmark trial by Goldstein et al. enrolled 532 men with organic, psychogenic, or mixed ED. At the optimal dose (mean ~50 mg), 69% of penetration attempts succeeded versus 22% with placebo (P<0.001), and 59% of intercourse attempts succeeded versus 15% with placebo 1. The International Index of Erectile Function (IIEF) erectile-function domain score improved by a mean of 8-9 points.

Vardenafil's registration program included a key trial by Porst et al. in men with diabetes-associated ED, a harder-to-treat population. At 20 mg, the IIEF erectile-function domain score improved by 7.8 points versus 1.4 for placebo (P<0.001). The penetration success rate (SEP2) was 64% versus 36%, and intercourse completion (SEP3) was 49% versus 23% 5.

A 2007 meta-analysis by Yuan et al. pooled 118 randomized controlled trials of PDE5 inhibitors and found no statistically significant difference in IIEF improvement between sildenafil and vardenafil in the general ED population 6. The weighted mean IIEF-EF improvement was 8.2 points for sildenafil and 7.8 points for vardenafil, with overlapping confidence intervals. Response rates clustered between 60-70% for both drugs in non-diabetic cohorts.

A preference-based crossover study by Rubio-Aurioles et al. (2006) randomized 1,057 men to sildenafil then vardenafil or the reverse sequence. Roughly 37% preferred sildenafil, 34% preferred vardenafil, and the remainder had no preference. The difference was not statistically significant 7. The finding tells us something practical: about one-third of men who do not respond optimally to one drug will do better on the other.

Pharmacokinetics: Onset, Duration, and Food Effects

Sildenafil reaches peak plasma concentration (Tmax) in approximately 60 minutes on an empty stomach. A high-fat meal delays Tmax by roughly 60 additional minutes and reduces Cmax by 29% 8. Its elimination half-life is 3-5 hours, giving a clinical duration of roughly 4-6 hours.

Vardenafil tablet also hits Tmax near 60 minutes fasted. A high-fat meal reduces its Cmax by 18-50%, depending on the fat content 3. The half-life is 4-5 hours, producing a similar 4-6 hour window. The Staxyn orally disintegrating tablet dissolves on the tongue without water and reaches Tmax slightly faster (~45 minutes), though bioavailability differs from the standard tablet, so the two formulations are not interchangeable milligram-for-milligram.

Both drugs should ideally be taken on an empty stomach or after a light meal for the most predictable onset. Neither drug provides next-day residual efficacy the way tadalafil (Cialis) does with its 17.5-hour half-life.

Dr. Arthur Burnett, Professor of Urology at Johns Hopkins, has stated: "PDE5 inhibitors within the same half-life range are clinically interchangeable for most patients, and the choice often reduces to which side-effect profile the patient tolerates best" 9.

Side Effects: Where the Drugs Diverge

The shared mechanism means overlapping adverse-event profiles. Headache, flushing, nasal congestion, and dyspepsia appear with both drugs at comparable rates. Pooled trial data show headache in 16% of sildenafil users and 15% of vardenafil users, flushing in 10% versus 11%, and dyspepsia in 7% versus 4% 6.

The divergence shows up in two areas.

Visual disturbance. Sildenafil's PDE6 cross-reactivity produces transient blue-green color tinge, increased brightness perception, or blurred vision in 3-11% of users at 100 mg 4. Vardenafil causes these effects in <2% of users. For men who find the visual side effects bothersome (particularly those who drive at night or operate equipment), this difference is clinically relevant.

QTc prolongation. Vardenafil produces a small, dose-dependent increase in QTc interval, averaging 8 ms at the 10 mg dose and 10 ms at 80 mg (supratherapeutic). The FDA label warns against vardenafil use in patients with congenital long QT syndrome and recommends caution with Class IA or Class III antiarrhythmics 10. Sildenafil does not carry this specific cardiac warning. For men on sotalol, amiodarone, or dofetilide, sildenafil is the safer PDE5 choice.

Priapism risk is a class effect, exceedingly rare (<0.1%) with both drugs. Both are contraindicated with nitrates and require 4-hour dose separation from tamsulosin or other alpha-blockers.

Practical Guide to Switching

Switching from sildenafil to vardenafil (or back) does not require a washout period. Both drugs clear to negligible plasma levels within 24 hours. The American Urological Association (AUA) guidelines on ED management recommend trialing a PDE5 inhibitor at maximum dose on at least 6-8 separate occasions before declaring it ineffective 11. A man who used sildenafil 50 mg twice and felt nothing has not had an adequate trial.

Dose equivalence. No formal dose-conversion table exists, but clinical consensus and prescribing-information pharmacology support this starting framework:

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

Dr. Irwin Goldstein, Director of Sexual Medicine at Alvarado Hospital, has noted: "When a patient has partial response to one PDE5 inhibitor, switching to another within the same half-life class succeeds roughly 30-40% of the time. This makes the switch a reasonable step before escalating to intracavernosal injections or vacuum devices" 12.

When to switch. Consider switching if:

1. Side effects are bothersome at effective doses (e.g., sildenafil causes intolerable visual disturbance; switch to vardenafil).
2. Efficacy is partial after 6-8 attempts at maximum dose with proper technique (empty stomach, adequate sexual stimulation, appropriate timing).
3. A formulation advantage matters. Vardenafil Staxyn ODT requires no water, which some men prefer for discretion.
4. Drug interactions change. A new prescription for an antiarrhythmic may favor sildenafil over vardenafil.

When switching is unlikely to help. If the underlying issue is severe arterial insufficiency (e.g., penile Doppler peak systolic velocity <25 cm/s), switching between two short-acting PDE5 inhibitors with similar half-lives rarely changes outcomes. In such cases, trying tadalafil daily 5 mg or moving to second-line therapy (intracavernosal alprostadil, vacuum erection device) is more productive 11.

Cost and Access Considerations

Sildenafil went generic in the United States in December 2017 after Pfizer's patent expired. Generic sildenafil 50 mg costs $0.50-3.00 per tablet at most pharmacies with a GoodRx-type discount. Brand Viagra is rarely dispensed.

Vardenafil's composition-of-matter patent expired in 2018, and generic vardenafil tablets are available at $2-8 per tablet. The Staxyn ODT formulation has a separate patent and remains more expensive, typically $15-30 per tablet without insurance. Not all formularies cover ED drugs, and prior authorization requirements vary.

For men on a budget, generic sildenafil is typically the cheapest first-line option. The cost difference is real. A man using 8 tablets per month could spend $4-24 on generic sildenafil versus $16-64 on generic vardenafil. If sildenafil works and is tolerated, cost alone is a valid reason to stay 13.

Special Populations

Diabetes. Both drugs are less effective in diabetic ED. Porst et al. showed vardenafil 20 mg produced a 49% intercourse success rate in diabetic men, compared to 63-69% in general-population sildenafil trials 5 1. Goldstein et al. reported sildenafil efficacy in a diabetic subgroup with similar magnitude reductions. No convincing data show one agent outperforms the other in diabetes specifically. Titrate to maximum dose and optimize glycemic control (HbA1c <7%) before switching 14.

Post-prostatectomy. Nerve-sparing radical prostatectomy patients have reduced nitric oxide signaling. Response rates to PDE5 inhibitors drop to 35-50% depending on nerve-sparing quality. Small studies suggest nightly low-dose PDE5 therapy may aid cavernosal rehabilitation, though the 2018 AUA guidelines note that evidence for penile rehabilitation remains mixed 11. Neither sildenafil nor vardenafil has demonstrated superiority in this context.

Mild hepatic impairment. Both drugs are hepatically metabolized via CYP3A4. The vardenafil label recommends starting at 5 mg in moderate hepatic impairment (Child-Pugh B). Sildenafil's label does not mandate dose reduction for mild impairment but recommends caution and a 25 mg starting dose in moderate impairment 8.

CYP3A4 inhibitors. Ketoconazole, itraconazole, ritonavir, and clarithromycin all increase exposure to both drugs. The vardenafil label caps dosing at 2.5 mg per 72 hours with ritonavir. Sildenafil should not exceed 25 mg per 48 hours with strong CYP3A4 inhibitors 10. Men on protease inhibitors should have their PDE5 dose adjusted by the prescribing clinician before any switch.

Which Drug to Start With

For most men with newly diagnosed ED, generic sildenafil 50 mg is the practical first choice. It has the longest track record (FDA-approved 1998), the lowest generic cost, and the broadest prescriber familiarity.

Vardenafil becomes the better starting option in three scenarios: the patient reports visual sensitivity or has a history of retinal disease (lower PDE6 cross-reactivity), the patient wants an ODT formulation, or the patient had partial but not zero response to sildenafil and wants to try a different short-acting agent before moving to tadalafil.

The AUA 2018 guidelines do not rank one PDE5 inhibitor above another. The recommendation is to trial one agent adequately (6-8 attempts, maximum tolerated dose, proper conditions) before switching or escalating 11.

If both sildenafil and vardenafil fail after adequate trials, tadalafil 5 mg daily offers a longer half-life and continuous drug exposure. If all oral PDE5 inhibitors fail, second-line options include intracavernosal alprostadil (10-20 mcg), vacuum erection devices, or surgical implant referral.