Viagra is the trade name, the generic name of the drug is sildenafil citrate tablets, it is a kind of erectile dysfunction medication, the lack of effect after taking sildenafil citrate tablets may be related to the taking of the drug dosage is too small, lack of sexual stimulation, the existence of organic erectile dysfunction and other factors. 1. Too small a dose of medication: when patients take sildenafil citrate tablets below the normal dosage, the dose of medication is too small, which may affect the therapeutic effect, so as to fail to achieve the desired therapeutic effect. 2. Lack of sexual stimulation: after taking sildenafil citrate tablets, if men lack sexual stimulation, it may also lead to the penis not being able to have a normal erection. 3. Existence of organic erectile dysfunction: when men have venous or arterial erectile dysfunction, such as smooth muscle dysfunction, penile arterial atherosclerosis, etc., it may make the cavernous body of the penis unable to obtain sufficient blood supply, thus appearing to be ineffective after taking the medicine. It is recommended that patients strictly respect the doctor’s instructions for the correct use of medication, if there is no effect after the use of medication to consult a doctor in time to check whether there is an organic lesion; in addition, sildenafil is prohibited from combining with nitrate drugs, women, people who are allergic to the product, children, and people who are not suitable for sex in recent cardiovascular status are prohibited from using this drug.