By controlling the enzyme, sildenafil helps to maintain an erection after the penis is stroked. If you have an erection that lasts for more than 4 hours, you should go to the nearest hospital emergency department. This product is available in the following dosage forms: Tablet Suspension There is a problem with information submitted for this request. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. Do not take it more than 3 times a day. The penis is one of the areas where this enzyme works.Erectile dysfunction is when a man has difficulty getting an erection or keeping it long enough for sex. It’s also known as ED or impotence. It happens when not enough blood flows to the penis, preventing an erection. The frequency increases with age, affecting about 50% of men over age 40*; however younger men may also experience ED. 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. Sildenafil tablets for erectile dysfunction come in different strengths ranging from 25mg to mg. The usual dose is 50mg, when you need it. Do not take it more than once a day. The dose can be increased to mg or decreased to 25mg depending on its effect. Take sildenafil up to 4 hours before you want to have sex.