Cialis stays in your system for up to 36 hours, and you can take it daily. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. You should make an appointment with your doctor to get a prescription before trying to buy Viagra. Because they are the same class of medications, they have similar side effects and effectiveness. Your healthcare provider can help you determine the right dosage for you.VIAGRA is supplied as blue, film-coated, rounded-diamond-shaped tablets containing sildenafil citrate equivalent to 25 mg, 50 mg, or mg of sildenafil. Tablets are debossed with PFIZER on one side and VGR25, VGR50 or VGR on the other to indicate the dosage strengths. Close. Vision changes. Muscle pain. But there are some differences. With Viagra, there is a higher risk of vision changes than with Cialis. Among people who take Viagra or sildenafil, 3% get blue vision that lasts a few hours. On the other hand, Cialis has a slightly higher risk of muscle pain than Viagra and vardenafil. Headache, flushing, and upset stomach are common Viagra side effects. These effects are usually mild and often resolve on their own. While rare, Viagra can cause an erection that lasts longer than 4 hours (priapism). This can result in permanent damage if left untreated. Vision changes are also possible.