When hypertension occurs in the lungs, the heart must work harder to pump enough blood through the lungs. Although not all of these side effects may occur, if they do occur they may need medical attention. Resources and support See the Healthy Male site for more information about erectile dysfunction and its treatment. If you have an erection that lasts more than 4 hours, get medical help right away. If you will be taking this medicine for pulmonary arterial hypertension, your doctor will want to check your progress at regular visits. If you need emergency medical care for a heart problem, it is important that your doctor knows when you last took sildenafil.Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash. 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.