Its use is not studied in patients with severe hepatic impairment. The onset of action can occur within 30 minutes, while the duration of the effect can last up to 18 hours. Patients taking concomitant inhibitors of CYP3A4 may benefit from initiation at the lower dosage. Some barriers to successful sildenafil therapy in the treatment of erectile dysfunction include cost and social stigma surrounding erectile dysfunction. If you forget to take it If you have pulmonary hypertension and you forget to take a dose of sildenafil, take it as soon as you remember unless it’s nearly time for your next dose.The normal dose when starting Viagra is 50 mg, as needed. The maximum dose of Viagra is mg each day. Doses above the maximum dose, such as mg per day, will increase the risk of side effects. Strengths: 25 mg, 50 mg, mg; Brand: Viagra. Form: oral tablet; Strengths: 25 mg, 50 mg, mg; Adult dosage (ages 18–64 years) Typical dosage: 50 mg, taken as needed about 1 hour before. Like Viagra, certain forms of sildenafil are approved to treat ED in males* ages 18 years and older. For example, sildenafil mg, mg, and mg tablets are approved for this use.