Viagra generally refers to sildenafil citrate tablets. How long it lasts once is determined by each individual’s constitution and is around 30 minutes to 4 hours. Sildenafil citrate tablets are mainly used to improve erectile dysfunction in men. It is generally recommended to be taken as needed about 1 hour before sexual activity, but can be taken any time between 30 minutes and 4 hours before sexual activity. It should be noted that the drug works only under stimulating conditions. Typically, men take this product after thirty minutes and it remains effective for about four hours, depending on the individual’s constitution. However, some studies have shown that the effects of sildenafil are shown to be weaker after 2 hours, individual differences still exist and each person may react differently. Possible adverse reactions to the drug include dizziness, headache, and indigestion. It should be noted that concurrent use of this product is prohibited in patients taking any form of organic nitrates or organic nitrites. In addition, it should be noted that Sildenafil should be taken in accordance with the recommendations of the doctor’s advice and drug instructions, and should not be used blindly on its own. Patients should strengthen the exercise, pay attention to the diet and regular work and rest, long-term dependence on drugs is not recommended.