Continue to take sildenafil even if you feel well. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food. If you become pregnant while taking sildenafil, call your doctor. Make sure you have discussed any other medications you are currently taking or plan to take before starting sildenafil with your doctor and pharmacist. Tell your doctor if sildenafil is not working well or if you are experiencing side effects. Take sildenafil exactly as directed. Other uses for this medicine This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information.Brand-name vs. generic Reducing long-term drug costs Financial assistance Next steps Viagra (sildenafil) is a brand-name oral tablet prescribed for erectile . 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. This means that 4 hours after you take a dose, your body has cleared half of the dose. In most cases, it takes about 5 half-lives for your body to completely remove a drug from your system. So.