If your doctor is concerned your ED is an indicator of a more serious problem, they may send you to a specialist who can run some additional tests. 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. If you are taking sildenafil for erectile dysfunction, you are unlikely to miss a dose since this medication is taken as needed, not on a regular dosing schedule. If you experience chest pain, dizziness, or nausea during sexual activity, call your doctor immediately and avoid sexual activity until your doctor tells you otherwise. These are commonly prescribed for an enlarged prostate or male pattern baldness. These medicines are called PDE5 inhibitors.FAQs What to consider Cost Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. . Below are details about Viagra dosages commonly used to treat erectile dysfunction (ED).. Viagra form. Viagra comes as tablets that you swallow. Viagra strengths. Viagra tablets . Your prescription may have been filled with a generic version instead of brand-name VIAGRA. Follow the steps below to help ensure you receive the brand name your doctor .