Viagra (sildenafil) can be used by patients who experience repeated inability to have sex. This drug is very efficient and works in 80% of all cases. Just like with any other medicine it doesn't work for a very small percentage of people, although in most cases this has to do with individual factors, such as certain health conditions the person has or drugs they are taking. These factors can affect the efficiency of Viagra and render your treatment less efficient. Once you have taken your dosage of Viagra the effects are likely to last for 4-5 hours, the period largely depends on the patient's age, health condition, habits and a number of other factors. You are unlikely to be prescribed Viagra for regular use, this is why there is no reason to worry about missing your dose. Viagra should be taken as needed about an hour before you are planning to have sex, with some food or without. However, remember that it is not recommended to take Viagra with fatty meals, as they can slow down the absorption of this medicine, as a result of which you will not feel the full benefits of taking Viagra and the onset of the effects will be delayed.