Cialis vs Vardenafil (Levitra/Staxyn): What to Do When One Fails

How Tadalafil and Vardenafil Work: The Same Target, Different Profiles

Both drugs block phosphodiesterase type 5 (PDE5), the enzyme that breaks down cyclic GMP in penile smooth muscle. Raised cyclic GMP relaxes smooth muscle, widens sinusoidal spaces, and allows blood to fill the corpus cavernosum. Sexual stimulation is still required; neither drug produces an erection without it. Read the FDA label for tadalafil.

Receptor Selectivity Differs

Vardenafil is roughly 10-fold more selective for PDE5 over PDE6 than sildenafil, which may explain its lower rate of visual side effects. Tadalafil is highly selective for PDE5 over PDE11, an isoform expressed in testicular and cardiac tissue, a distinction that became relevant in the BPH indication. The clinical meaning of these selectivity differences in head-to-head ED trials is modest, but they do influence tolerability profiles in practice.

Pharmacokinetic Comparison

Tadalafil reaches peak plasma concentration (Tmax) in 2 hours and has a half-life of approximately 17.5 hours, giving a therapeutic window of up to 36 hours. The FDA approved tadalafil 5 mg for once-daily use specifically because this long half-life supports continuous dosing. Vardenafil reaches Tmax in 30 to 120 minutes and has a half-life near 4 to 5 hours. A high-fat meal slows vardenafil absorption and can push Tmax out by up to 60 minutes, a factor that matters for on-demand use. Staxyn, the orally disintegrating tablet formulation, achieves a higher Cmax than conventional vardenafil 10 mg film-coated tablets, even though the molecules are identical.

Clinical Efficacy: What the Trials Actually Show

Tadalafil Key Data

The phase 3 registration trials for tadalafil showed International Index of Erectile Function (IIEF) erectile function domain scores rising from baseline means around 12 to roughly 25 at 20 mg doses, compared with minimal placebo change. Brock et al. (J Urol 2002, N=179 in the dose-escalation arm) reported that 81 percent of tadalafil-treated attempts resulted in successful intercourse versus 35 percent for placebo. The trial included men with diabetes and radical prostatectomy history, populations that are historically harder to treat, which strengthens the generalizability of those numbers.

Vardenafil Key Data

Porst et al. (Int J Impot Res 2003) enrolled men with mild-to-severe ED in a double-blind, placebo-controlled design. Vardenafil 20 mg produced successful intercourse in 75 percent of attempts, compared with 52 percent for 10 mg and 32 percent for placebo. IIEF erectile function domain scores improved by 8.0 points over placebo at the 20 mg dose. Men with diabetes in that cohort still showed statistically significant improvement, though absolute response rates were lower than in non-diabetic subgroups, consistent with the broader PDE5 inhibitor literature on diabetes-related ED. A meta-analysis in the European Urology journal confirmed comparable efficacy across the three first-generation PDE5 inhibitors when doses were optimized.

Head-to-Head Evidence

No large, adequately powered, prospectively randomized head-to-head trial has proven one agent categorically superior for the general ED population. A crossover study published in the International Journal of Impotence Research found that patient preference varied significantly, with approximately 29 percent preferring tadalafil, 27 percent sildenafil, and roughly equal distributions reflecting individual pharmacodynamic variability rather than class superiority. This finding supports a trial-and-switch strategy rather than declaring a universal winner.

Defining True Treatment Failure

Why "Failure" Is Often Premature Abandonment

An estimated 50 to 60 percent of men who report that a PDE5 inhibitor "doesn't work" have never taken an optimal dose under ideal conditions. The AUA Erectile Dysfunction Guideline specifies that a drug should be tried at the maximum tolerated dose on at least 4 to 6 occasions with adequate sexual stimulation before concluding non-response. Suboptimal trials include taking vardenafil after a high-fat meal, attempting intercourse without sufficient foreplay, or using tadalafil on demand at 10 mg when 20 mg is tolerated and indicated.

Organic vs Psychogenic Failure

Men with severe arteriogenic disease, bilateral cavernous nerve injury after radical prostatectomy, or uncontrolled diabetes (HbA1c above 9 percent) have lower absolute response rates to all PDE5 inhibitors. One study of post-prostatectomy ED found that tadalafil 20 mg on demand produced return of intercourse ability in only 52 percent of nerve-sparing patients and 15 percent of non-nerve-sparing patients at 24 weeks. When organic burden is high, switching within class may add marginal benefit; escalation to intracavernosal injection therapy or vacuum erection devices becomes the more evidence-aligned step.

Patient Education as the First Intervention

Before any switch, a clinician should review administration technique, timing relative to meals and sexual activity, alcohol intake (which reduces erectile response), and concurrent medications. Nitrate use in any form remains an absolute contraindication for both tadalafil and vardenafil, and men taking nitrates for chest pain should not attempt a PDE5 switch without cardiology clearance.

When to Switch from Tadalafil to Vardenafil

Scenario 1: Tadalafil Side Effects Drive the Switch

Tadalafil produces more myalgia and back pain than other PDE5 inhibitors, reported in 3 to 7 percent of men in registration trials. This effect results from PDE11 inhibition in skeletal muscle. The FDA label for tadalafil lists back pain and myalgia as common adverse events. For a man who achieves erections on tadalafil but stops the drug because of muscular discomfort, vardenafil is a reasonable alternative. Vardenafil's PDE11 selectivity is higher, and myalgia rates in its trials were below 1 percent.

Scenario 2: Daily Tadalafil Is Inconvenient or Uninsured

At 2.5 to 5 mg daily, tadalafil costs significantly more out-of-pocket per month than on-demand vardenafil used 4 to 8 times per month. A man whose lifestyle involves infrequent but predictable sexual activity may find vardenafil 10 to 20 mg taken 60 minutes before intercourse more cost-effective. Generic vardenafil became available in the United States after 2018, reducing the price differential substantially.

Scenario 3: Tadalafil Produces Insufficient Rigidity

If a man reports partial erections on tadalafil 20 mg after 6 attempts with adequate stimulation, vardenafil's potentially faster onset and different receptor-binding kinetics offer a rational trial. Vardenafil's IC50 for PDE5 is approximately 0.14 nM, compared with tadalafil's 0.94 nM, meaning vardenafil binds PDE5 with greater potency in vitro, though this does not translate to a predictable clinical advantage at standard doses for all men.

When to Switch from Vardenafil to Tadalafil

Scenario 1: Timing and Spontaneity Are the Problem

A man who reports that vardenafil "wears off" before intercourse occurs, or who finds the 60-minute planning window stressful, is an ideal candidate for tadalafil. Daily tadalafil 5 mg eliminates the need for pre-planning entirely. A 2008 crossover study (N=215) found that men assigned to daily tadalafil reported significantly higher scores on the IIEF sexual encounter profile question 3 ("Did your erection last long enough?") compared with their own on-demand sildenafil period, with P<0.001 for the difference. The spontaneity advantage is real and measurable.

Scenario 2: Cardiac Rhythm Concerns

Vardenafil causes mild QTc prolongation of approximately 8 milliseconds at the 10 mg dose and 10 milliseconds at 80 mg in pharmacodynamic studies. The FDA label for vardenafil contraindicates co-administration with Class IA antiarrhythmics (quinidine, procainamide) and Class III antiarrhythmics (amiodarone, sotalol). Tadalafil does not carry the same QTc signal and is preferred when a man is on antiarrhythmic therapy, provided no nitrates are involved. A pharmacokinetic review in Clinical Pharmacokinetics confirmed tadalafil's absence of QT effects at therapeutic doses.

Scenario 3: Co-existing BPH Symptoms

Tadalafil 5 mg daily is the only PDE5 inhibitor with an FDA-approved indication for both ED and lower urinary tract symptoms secondary to benign prostatic hyperplasia. The LVHJ phase 3 BPH trial demonstrated a 4.9-point improvement in International Prostate Symptom Score (IPSS) versus 2.3 points for placebo (P<0.001) at 12 weeks. Men who report both ED and urinary symptoms should consider a switch to tadalafil specifically because of this dual benefit.

Cross-Class Rescue: What Happens After a Structured Switch

Success Rates in Cross-Over Trials

The best evidence for within-class switching comes from two sources. First, a prospective study by Hatzimouratidis et al. showed that men who failed sildenafil and crossed to tadalafil had a 63 percent response rate when "failure" was properly defined and dosing was optimized. Second, a retrospective cohort analysis found that approximately 55 percent of men who discontinued one PDE5 inhibitor due to inefficacy responded to a second agent from the same class. A Cochrane systematic review of PDE5 inhibitors noted that the evidence base for within-class switching is based largely on observational data and small crossover trials, meaning the 50 to 70 percent success figure cited in guidelines carries moderate, not high, certainty.

Why Switching Can Work Despite Shared Mechanism

The clinical paradox of same-class switching yielding rescue responses is partly explained by individual PDE5 isoform expression variability, differential protein binding, and differences in onset kinetics that match better with a specific man's psychogenic arousal window. Administration errors corrected during counseling before the switch account for a meaningful proportion of apparent "rescues." Separating true pharmacodynamic rescue from corrected-administration rescue requires controlled conditions that most real-world switch studies do not provide.

Staxyn as an Alternative Formulation

For men who tried oral vardenafil tablets without success, Staxyn (vardenafil 10 mg orally disintegrating tablet) provides a different absorption profile. Pharmacokinetic data show Staxyn produces a 28 percent higher Cmax than the film-coated 10 mg tablet under fasted conditions. This is not a dose increase, but faster peak concentration may improve on-demand efficacy for men with delayed absorption patterns. The FDA approved Staxyn in 2010. Staxyn should not be taken with liquid; it dissolves on the tongue.

Safety, Contraindications, and Drug Interactions

Nitrate Contraindication: Non-Negotiable

Both tadalafil and vardenafil are absolutely contraindicated with organic nitrates (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate) and nitric oxide donors. The combination causes severe, potentially fatal hypotension. The FDA has not approved any protocol for co-administration, and the washout period required before a nitrate can safely follow a PDE5 inhibitor is at least 48 hours for tadalafil given its long half-life, compared with 24 hours for vardenafil.

Alpha-Blocker Interactions

Alpha-blockers used for BPH (tamsulosin, alfuzosin, doxazosin) can potentiate hypotension with both PDE5 inhibitors. Tadalafil's label recommends that patients be on a stable alpha-blocker dose before starting tadalafil, and initiating at the lowest dose (2.5 mg) is standard practice. Tamsulosin 0.4 mg is the alpha-blocker with the least hemodynamic interaction data, and it is generally considered safer to combine with tadalafil than non-selective alpha-blockers.

CYP3A4 Drug Interactions

Both tadalafil and vardenafil are metabolized primarily by CYP3A4. Strong CYP3A4 inhibitors, including ketoconazole, ritonavir, and clarithromycin, can raise plasma concentrations substantially. For tadalafil, ritonavir co-administration increases tadalafil AUC by 124 percent. The practical instruction: men on HIV protease inhibitors should not exceed tadalafil 10 mg every 72 hours or vardenafil 2.5 mg every 72 hours, per their respective FDA labels.

Cardiovascular Risk Assessment Before Prescribing

The Princeton III Consensus published in the American Journal of Cardiology categorizes men into low, intermediate, and high cardiovascular risk before PDE5 inhibitor prescribing. Low-risk men (controlled hypertension, asymptomatic, fewer than three CAD risk factors, NYHA class I) can start either drug without further cardiac workup. Intermediate or high-risk men need physician-supervised exercise testing or cardiology clearance. This risk-stratification step is mandatory before any PDE5 switch, not just the first prescription.

Dosing Protocol for a Structured Switch

Tadalafil Dosing Ladder

Start tadalafil at 10 mg on demand. If three attempts produce partial response, escalate to 20 mg. If on-demand use is inconvenient or insufficient, convert to daily 5 mg and allow 2 to 4 weeks for steady-state efficacy. The FDA-approved dose range is 5 to 20 mg. Renal impairment (CrCl <30 mL/min) limits the maximum dose to 5 mg; hepatic impairment (Child-Pugh B) limits use to 10 mg.

Vardenafil Dosing Ladder

Start vardenafil at 10 mg taken 25 to 60 minutes before sexual activity. Escalate to 20 mg if response is insufficient after three to four attempts. The maximum dose is 20 mg per day. Men over 65 should start at 5 mg. For Staxyn (ODT formulation), 10 mg is the only available dose and is not interchangeable with Levitra 10 mg film-coated tablets on a milligram-for-milligram bioavailability basis.

The 4-6-Attempt Rule

The AUA's 2018 erectile dysfunction guideline states directly: "Clinicians should consider optimizing PDE5i therapy before diagnosing treatment failure, including instructions on proper use, dose escalation to the maximum tolerated dose, and allowing at least four to six attempts." Documenting this optimization in the medical record matters before any switch or escalation to second-line therapy. The full guideline text is available at AUA.

Second-Line Options When Both Drugs Fail

When both tadalafil at 20 mg on demand and vardenafil at 20 mg on demand fail after proper optimization, the clinical pathway moves to second-line options. These include intracavernosal injection (ICI) therapy with alprostadil, papaverine, or phentolamine combinations; intraurethral alprostadil (MUSE); and, for men with severe vascular or neurogenic ED, penile prosthesis implantation. A meta-analysis in the Journal of Sexual Medicine found that ICI therapy with trimix (papaverine-phentolamine-alprostadil) produced erections sufficient for intercourse in over 87 percent of men who failed oral PDE5 inhibitors, including men with post-prostatectomy and diabetic ED. Referral to a urologist specializing in sexual medicine is appropriate at this stage.

Testosterone deficiency can also blunt PDE5 inhibitor response. A trial by Shabsigh et al. found that men with hypogonadism (total testosterone <300 ng/dL) who failed sildenafil responded at a 60 percent rate when testosterone replacement was added, versus 11 percent who continued sildenafil alone without testosterone correction. Checking a morning total testosterone before labeling any man a "PDE5 inhibitor non-responder" is standard of care.