One capsule of Viagra usually works for 4 hours, but studies have shown that its effects are weaker after 2 hours, but there are individual differences and each person’s reaction to Viagra may be different. The generic name of Viagra is sildenafil citrate tablets, which are used to improve erectile dysfunction in men. It is generally recommended to take it one hour before sex, because after taking Viagra, the blood concentration peaks in 0.5-2 hours, and in most patients the peak time is in one hour. If there are other reasons for not being able to take it on time, studies have also shown that it can be taken between 0.5-4 hours of sex. In addition, the drug must be taken under conditions of sexual stimulation in order for it to be effective. The specific usage of Viagra should be taken in accordance with the doctor’s advice and the recommendations of the drug instructions. It is not recommended to rely on the drug for a long time.