Viagra is the trade name, generic name is sildenafil citrate, after eating an hour did not feel may be the drug has not yet taken effect, lack of sexual stimulation, the drug dose is too small and other reasons. 1. The drug has not yet worked: this drug according to the instructions recommended usage, should be taken about 1 hour before sexual activity on demand, some people may be slower to take effect, so the drug may not have taken effect. In addition, if the high-fat diet, may affect the absorption, prolong the effect of time. 2. Lack of sexual stimulation: Taking sildenafil citrate requires some sexual stimulation to be effective. 3. Too small a dose of the drug: the dose of the drug may be too small to reach the therapeutic amount, so there is no response after 1 hour. This drug is suitable for the treatment of erectile dysfunction, etc. Adverse reactions such as headache, flushing and myalgia may occur after using the drug, which can be gradually relieved after stopping the drug in general. Patients with cardiovascular disease should use the drug with caution, and this drug should not be used in combination with drugs such as levocetirizine and nitrate at the same time. The use of this drug is prohibited for those who are allergic to this drug or the components of the formula, women, children. The specific dosage and usage of the above drugs should follow the doctor’s instructions, and should not be used on their own to avoid adverse reactions.