If you experience chest pain, dizziness, or nausea during sex, seek immediate medical help. Click here for an email preview. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine. Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure. Sildenafil may not be safe to take for people with certain medical conditions including: a past history of stroke or heart attack in the last 6 months retinitis pigmentosa an eye disease It can also interact with many other medicines, particularly nitrates used in heart disease , amyl nitrite and blood pressure medicines. Resources and support See the Healthy Male site for more information about erectile dysfunction and its treatment. If you have an erection that lasts for more than 4 hours, you should go to the nearest hospital emergency department.Viagra or generic sildenafil for ED should not be used in women. For women who are breastfeeding: Strengths: 25 mg, 50 mg, mg; Brand: Viagra. Form: oral tablet; Strengths: 25 mg. The recommended dosage of Viagra for erectile dysfunction (ED) is 50 mg, taken as needed about 1 hour before sexual activity. You can take it 30 minutes to 4 hours before sexual activity. And you. 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 consider possible drug interactions.