Viagra generally refers to sildenafil citrate tablets, and according to the dosing instructions the medication is taken on demand 1 hour before sexual activity, or anytime between 0.5 and 4 hours before sexual activity. Sildenafil Citrate Tablet is a medication used to treat erectile dysfunction in men. According to its dosing instructions, for most patients, taking one tablet with a dose of 50mg is recommended to take the drug as needed about 1 hour before activity, but it can also be taken at but any time within 0.5 to 4 hours before sexual activity. It is to be taken a maximum of 1 time per day. Sildenafil citrate tablets should be used regularly under the supervision of a doctor. Adverse reactions such as headache, flushing, indigestion, back pain, myalgia, nausea, and breathing difficulties may occur during the use of the drug. The drug is forbidden to be used in combination with nitrate drugs, and it is also forbidden for women, people who are allergic to the product, children, and people whose recent cardiovascular status makes them unfit for sexual intercourse.