It may be difficult to remain firm despite wanting to have sex. Sildenafil Viagra is used to treat erectile dysfunction impotence; inability to get or keep an erection in men. What Are the Symptoms of Erectile Dysfunction? Rarely reported side effects include: an erection that will not go away priapism. John’s wort. It’s available in 25 mg, 50 mg, and mg tablets and lasts up to 18 hours. Erectile dysfunction can be caused by a variety of factors , including age, cardiovascular problems, diabetes, and other conditions that affect nerve function or blood flow.What Happened When I Took Viagra Recreationally. 1. Viagra was effective for much longer than four hours. Shutterstock. Viagra comes in three different sizes: 25mg, 50mg, or mg. In the past, I’ve found that 25mg always did the job just fine. However, my journey begins with a bottle of the mg tabs which I’ve chosen to cut into thirds. Viagra Dosage and Use Tips for First-Timers. Viagra dosage for erectile dysfunction (ED) is usually milligrams (mg), but those over 65 may start at 25 mg. Your doctor will also need to. Typical dosing for Sildenafil (Viagra) The typical dose is to take 50 mg of sildenafil (Viagra) by mouth, about 1 hour before sex, as needed. The maximum recommended dose is mg. Don’t take sildenafil (Viagra) more than once daily (at any dose). Written by Brian Leonard, PharmD, BCACP, BCGP.