Ask your doctor if you have any questions. There are many safe, reliable online services offering ED prescriptions. Sildenafil should not be used with any other medicine or device that causes erections. If you experience chest pain, dizziness, or nausea during sex, seek immediate medical help. This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. The most common side effects of VIAGRA: headache; flushing; upset stomach; abnormal vision, such as changes in color vision such as having a blue color tinge and blurred vision; stuffy or runny nose; back pain; muscle pain; nausea; dizziness; rash.Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . 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. Price without insurance: starts at $6 per dose for 10 doses. Doses: 20, 25, 50, and mg. Supply: 30, 90, or days. Sildenafil is the generic form of Viagra and Revatio. It’s chemically.