What is the recommended dosing for sildenafil (Viagra) in a patient with erectile dysfunction, considering their medical history and potential interactions?

Sildenafil Dosing for Erectile Dysfunction

Start with sildenafil 50 mg taken approximately 1 hour before sexual activity, not more than once daily, and titrate to 100 mg if the response is suboptimal after at least 5 separate attempts, or reduce to 25 mg in elderly patients, those with hepatic/renal impairment, or when co-administered with CYP3A4 inhibitors. 1

Critical Pre-Treatment Safety Screening

Before prescribing any dose of sildenafil, you must verify the following absolute contraindications and risk factors:

• Nitrate use is an absolute contraindication due to life-threatening hypotension risk—ask explicitly about all forms of nitrates including sublingual, oral, transdermal, and recreational (e.g., "poppers") 1
• Assess cardiovascular fitness by asking if the patient can walk 1 mile in 20 minutes or climb 2 flights of stairs without symptoms 1
• Patients with ≥3 cardiac risk factors, moderate stable angina, recent MI (2-6 weeks), or NYHA Class II heart failure require cardiovascular assessment before treatment 1
• High-risk patients (unstable angina, recent MI/stroke within 2 weeks, NYHA Class III-IV heart failure, high-risk arrhythmias, hypertrophic obstructive cardiomyopathy) should defer sildenafil until cardiac condition is stabilized 1

Standard Dosing Algorithm

Initial Dose Selection

• Most patients aged 18-65 years: Start with 50 mg taken approximately 1 hour before sexual activity, not more than once daily 2
• Elderly patients (>65 years): Start with 25 mg to minimize adverse effects while maintaining efficacy 1, 2
• Patients with hepatic or renal impairment: Use 25 mg as the starting dose 2
• Patients on CYP3A4 inhibitors (ritonavir, saquinavir, ketoconazole, erythromycin, cimetidine): Reduce dose to 25 mg to avoid drug interactions 1, 2

Dose Titration Strategy

The European Urology Association notes that improvement in erectile function is dose-related for sildenafil, with 50 mg being more effective than 25 mg, though less additional benefit occurs when increasing from 50 mg to 100 mg 1. Despite this, titration is appropriate when:

• Increase to 100 mg when: The patient tolerates 50 mg well but erectile function remains suboptimal after at least 5 separate attempts with proper sexual stimulation, correct timing, and absence of modifiable factors 1
• Maintain or reduce dose when: The patient experiences significant adverse effects (headache, flushing, dyspepsia, nasal congestion, abnormal vision) or achieves satisfactory erectile function at the current dose 2, 3

Defining an Adequate Trial Before Declaring Failure

Before concluding that sildenafil is ineffective, patients must complete at least 5 separate attempts at the maximum tolerated dose—this single requirement converts up to 30% of apparent non-responders into successful responders 1. Many apparent failures are due to:

• Lack of adequate sexual stimulation (PDE5 inhibitors require arousal to work) 1
• Improper timing (taking medication with large or fatty meals reduces efficacy) 1
• Heavy alcohol use (impairs erectile function independent of medication) 1
• Unaddressed hormonal abnormalities (men with testosterone deficiency respond less robustly) 1
• Relationship issues with partner (psychological factors significantly impact response) 1

Special Population Considerations

Patients with Diabetes or Post-Prostatectomy

• These patients have more severe ED at baseline and may respond less robustly to sildenafil, but the medication remains effective 1
• Dose escalation to 100 mg is particularly appropriate in these groups when initial doses are well-tolerated but suboptimal 1

Patients with Testosterone Deficiency

• Men with hypogonadism respond less robustly to sildenafil alone 1
• Combining sildenafil with testosterone therapy is more effective than sildenafil alone in men with confirmed testosterone deficiency 1
• Check testosterone levels before declaring treatment failure 1

Elderly Patients (>70 Years)

• Sildenafil is highly effective and safe in elderly patients, with 69% reporting successful sexual intercourse attempts versus 35.5% with placebo 1
• Start with 25 mg and titrate based on response and tolerability 1

Pharmacokinetics and Timing

• Sildenafil is rapidly absorbed with peak plasma concentrations within 1 hour of administration 2
• Elimination half-life is 3 to 5 hours 2
• Take approximately 1 hour before sexual activity, not more than once daily 1, 2
• Avoid taking with large or fatty meals as this reduces efficacy 1

Adverse Effects and Tolerability

Higher doses of sildenafil are associated with greater risk for adverse effects, necessitating dose titration to balance efficacy and tolerability 1. Common adverse events include:

• Headache, flushing, dyspepsia, nasal congestion, and abnormal vision 2, 3
• These effects are typically transient and mild to moderate 2
• Serious adverse events occur in <2% of patients, with no significant difference from placebo 1

Ongoing Management and Follow-Up

• Periodic follow-up (ideally between 4 weeks and 6 months) should assess efficacy, side effects, cardiovascular status changes, new medications, and overall health status 1
• Before switching therapies, verify that the trial was adequate by addressing hormonal abnormalities, food or drug interactions, timing and frequency of dosing, adequacy of sexual stimulation, heavy alcohol use, and relationship issues 1
• If a patient on sildenafil requires emergency nitrate administration, wait 24 hours after the last sildenafil dose before giving nitrates, under close medical supervision 1

Common Pitfalls to Avoid

• Declaring treatment failure without completing at least 5 attempts at maximum dose is the most critical error 1
• Failing to verify adequate sexual stimulation—sildenafil requires arousal to work 1
• Not screening for heavy alcohol use, which impairs erectile function independent of medication 1
• Overlooking testosterone deficiency, which significantly reduces response to sildenafil alone 1
• Prescribing without explicitly asking about all forms of nitrate use 1