Sildenafil: Usage, Dosage Side Effects - Drugs.com It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex. Taking sildenafil alone will not cause an erection. You need to be sexually excited for it to work. The most common side effects are headaches, feeling sick, indigestion and dizziness. Viagra is a drug that helps...