Can a male patient with erectile dysfunction, currently taking Cialis (tadalafil) 10 mg per day, also use Viagra (sildenafil) if Cialis alone is ineffective?

Combining Cialis and Viagra is Contraindicated

No, a patient taking Cialis 10 mg daily should NOT add Viagra (sildenafil) to the regimen, as combining two PDE5 inhibitors significantly increases the risk of severe hypotension and other serious adverse effects without proven additional benefit. 1

The Evidence Against Combination Therapy

The American College of Physicians explicitly recommends optimizing the dose of a single PDE5 inhibitor rather than adding a second one, citing increased risk of severe hypotension and adverse effects when combining these medications. 1 This is a critical safety concern that supersedes any theoretical benefit from dual therapy.

What to Do Instead: Systematic Approach to Treatment Failure

Before declaring tadalafil ineffective, you must ensure an adequate trial has been completed:

Step 1: Verify Adequate Trial

• At least 5 separate attempts at maximum dose (20 mg for as-needed dosing or 5 mg for daily dosing) are required before concluding treatment failure. 1
• Note that tadalafil does NOT show dose-dependent improvement between 5 mg, 10 mg, and 20 mg, so increasing from 10 mg to 20 mg may not dramatically improve efficacy but should still be attempted for tolerability assessment. 1

Step 2: Address Modifiable Factors

Systematically evaluate and correct these common causes of apparent treatment failure: 1

• Sexual stimulation adequacy - PDE5 inhibitors require sexual arousal to work; incorrect use accounts for a large percentage of treatment failures 1
• Timing issues - Verify proper timing relative to sexual activity
• Heavy alcohol use - This impairs erectile function and must be addressed 1
• Hormonal abnormalities - Check testosterone levels (total testosterone, free testosterone, SHBG) 1
• Relationship issues - Partner dynamics can significantly impact treatment success 1
• Drug interactions - Review all medications, particularly CYP3A4 inhibitors 2

Step 3: Consider Testosterone Supplementation

• For men with confirmed testosterone deficiency, combining tadalafil with testosterone therapy is more effective than tadalafil alone. 1
• Men with testosterone deficiency respond less robustly to PDE5 inhibitors alone. 1
• Testosterone therapy alone is NOT effective monotherapy for ED. 1

Step 4: Switch to a Different PDE5 Inhibitor

If tadalafil truly fails after adequate trial and correction of modifiable factors:

• Switch to sildenafil (Viagra) or vardenafil as monotherapy - do not combine them. 1
• Sildenafil shows dose-dependent improvement (50 mg more effective than 25 mg), unlike tadalafil. 1, 2
• Start sildenafil at 50 mg, taken 1 hour before sexual activity, and can titrate to 100 mg if needed. 3
• An adequate trial of the new agent also requires at least 5 attempts at maximum dose. 2

Step 5: Consider Daily Tadalafil if Currently Using As-Needed Dosing

• Switching from as-needed to daily tadalafil (2.5-5 mg once daily) can restore normal erectile function in 39-40% of men who had incomplete response to as-needed PDE5 inhibitor therapy. 4
• Daily dosing eliminates timing concerns and provides spontaneity. 1

Critical Safety Reminders

• Absolute contraindication with nitrates - verify no nitrate use before any PDE5 inhibitor. 1, 2
• If emergency nitrates are needed, wait 24 hours after sildenafil or 48 hours after tadalafil. 1
• Higher cumulative PDE5 inhibitor doses increase risk of headache, dyspepsia, back pain, myalgia, and hypotension. 1

Common Pitfalls to Avoid

• Do not combine PDE5 inhibitors - this is the most critical error to avoid. 1
• Do not declare failure prematurely - most "failures" are inadequate trials or unaddressed modifiable factors. 1
• Do not ignore testosterone deficiency - this significantly limits PDE5 inhibitor response. 1, 2
• Do not forget cardiovascular risk assessment - ensure patient can perform moderate physical activity (walk 1 mile in 20 minutes or climb 2 flights of stairs). 1