Protein binding is independent of total drug concentrations. Your healthcare provider will discuss which dose is right for you. You can buy it from a store-based pharmacy or reputable online pharmacy with a prescription. What alternatives can I use for erectile dysfunction? The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Some people buy sildenafil online without a prescription. Both sildenafil and the metabolite have terminal half lives of about 4 hours.About sildenafil (Viagra) Sildenafil is a medicine used to treat erection problems (erectile dysfunction). Sildenafil is also sometimes used to treat pulmonary hypertension (high blood pressure in the blood vessels that supply the lungs). If you have erection problems sildenafil works by temporarily increasing blood flow to your penis when. VIAGRA may not be right for every man, but it has been shown to help approximately 4 out of 5 men get and keep erections hard enough for sex at doses of 50 mg or mg.* * Seventy-two percent, 80%, and 85% of the patients on 25 mg, 50 mg, and mg of VIAGRA, respectively, achieved erections hard enough for sex, compared to 50% on . Viagra is available in mg, mg, and mg doses and can be purchased in day and day supplies. Shipping is free, and your new medication can be sent discreetly to .