What side effects can this medication cause? 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. Take sildenafil at around the same times every day. Tadalafil Cialis. Recent heart or blood vessel problems. Take sildenafil as needed before sexual activity.Generic Viagra (sildenafil) is an alternative to the branded Viagra drug that can be used for treating erectile dysfunction (ED). It’s now sold by many manufacturers, including online. Viagra dosage is not based on weight, but your age can affect your dosage. Specifically, you may have a lower starting dosage than usual if you’re over age For example, a . Sildenafil (Viagra brand) only works following sexual stimulation. It works within 30 to 60 minutes, but may be taken up to four hours before sexual activity. Sildenafil (Revatio .