Viagra (Sildenafil) Dosing and Administration
For Erectile Dysfunction
Start with sildenafil 50 mg taken approximately one hour before sexual activity, with adjustment to 25-100 mg based on efficacy and tolerability. 1
Initial Dosing Strategy
- Begin at 50 mg taken one hour before anticipated sexual activity 1, 2
- Maximum frequency: once daily 2, 3
- Sexual stimulation is required for effectiveness 4
Dose Titration
- Increase to 100 mg if 50 mg provides insufficient efficacy 1, 5
- Reduce to 25 mg for elderly patients, those with hepatic/renal impairment, or patients taking CYP3A4 inhibitors (ritonavir, ketoconazole, erythromycin, cimetidine) 2
- Approximately 69% of men report improved erections with sildenafil versus 35.5% with placebo 1
Defining Treatment Adequacy
- Require at least 5 separate attempts at maximum dose before declaring treatment failure 4
- Common reasons for apparent failure include improper timing, lack of sexual stimulation, or modifiable factors 4
Important Clinical Consideration
- The 100 mg starting dose significantly reduces anxiety about the next intercourse attempt (56% felt no anxiety vs 39% with 50 mg) and improves treatment satisfaction without increasing adverse events 5
- However, guideline recommendations still favor the 50 mg starting dose for initial therapy 1
For Pulmonary Arterial Hypertension (PAH)
The FDA-approved dose is 20 mg three times daily, administered 4-6 hours apart. 6
Dosing Specifications
- 20 mg orally three times daily (TID) 7, 6
- Space doses 4-6 hours apart 6
- Higher doses are NOT recommended - no greater efficacy was demonstrated with doses above 20 mg TID 6
Pediatric PAH Dosing
- Age <1 year: 0.5-1 mg/kg three times daily 7
- Weight <20 kg: 10 mg three times daily 7
- Weight >20 kg: 20 mg three times daily 7
- Avoid higher dosing in children - the STARTS-2 study showed greater mortality risk with high-dose sildenafil monotherapy in pediatric IPAH 7
- Delay use in extremely preterm infants until retinal vascularization is established 7
Critical Safety Contraindications
Absolute Contraindication
Sildenafil is absolutely contraindicated with any form of nitrate medication due to risk of potentially fatal hypotension. 6, 8, 3
- This includes nitroglycerin (tablets, patches, sprays, ointments), isosorbide mononitrate/dinitrate, and "poppers" (amyl nitrate/nitrite) 6
- If emergency nitrates are needed: wait 24 hours after sildenafil before administering nitrates under close medical supervision 4
Cardiovascular Risk Stratification
- Low-risk patients (can safely receive sildenafil): asymptomatic CAD with <3 risk factors, controlled hypertension, mild stable angina, successful coronary revascularization, uncomplicated past MI, mild valvular disease, CHF NYHA class I 4
- Refer to cardiology before prescribing if cardiovascular risk is indeterminate or patient cannot perform moderate physical activity 4, 8
- Use caution within 6 months of acute MI or stroke, in cardiac failure, and with unstable angina 3
Common Adverse Effects
For Erectile Dysfunction
- Headache, flushing, dyspepsia, nasal congestion, abnormal vision 2, 3
- Effects are typically transient and mild to moderate 2
For PAH
- Headache, nasal congestion, flushing, agitation, hypotension 7
- Vision and hearing loss are potential concerns 7
- Priapism (rare but serious) 7
Serious Adverse Events to Monitor
- Sudden vision loss (NAION) - instruct patients to report immediately 6
- Sudden hearing loss - instruct patients to report immediately 6
- Erections lasting >4 hours - require immediate medical attention 6
Drug Interactions Requiring Dose Adjustment
- CYP3A4 inhibitors (ritonavir, ketoconazole, itraconazole, erythromycin) require dose reduction to 25 mg 2
- Alpha-blockers: can be combined but monitor for hypotension 4
- Antihypertensive medications: modest, transient blood pressure reductions occur but generally well-tolerated 8
Special Populations
Hepatic/Renal Impairment
Diabetes and Post-Prostatectomy
- These patients have more severe ED at baseline and may respond less robustly 4
- Consider combination with testosterone therapy if testosterone deficiency is present 4
Testosterone Deficiency
- Combining sildenafil with testosterone therapy is more effective than sildenafil alone in men with both ED and testosterone deficiency 4
- Testosterone alone is not effective monotherapy for ED 4
Follow-Up Monitoring
- Periodic assessment should verify cardiovascular health hasn't changed, review new medications, assess treatment effectiveness, and evaluate side effects 1, 4
- Before switching therapies, address potential causes of inadequate response: hormonal abnormalities, drug interactions, timing/frequency issues, adequacy of sexual stimulation, alcohol use, relationship issues 4