The most familiar drug for male sexual dysfunction is Viagra. Viagra (Chinese name: 万艾可) has set off a blue whirlwind around the world. However, many people choose to buy it themselves in pharmacies or online due to psychological barriers. The drugs bought through these two channels may be irregular drugs. The effect of irregular drugs will be discounted after use, which will increase the psychological burden of patients. The mechanism of action and the use of precautions and contraindications are explained. The physiological mechanism of penile erection includes the release of nitric oxide in the penile corpus cavernosum during sexual stimulation, which then activates an enzyme called GC, leading to a rise in cGMP content, causing the smooth muscle in the penile corpus cavernosum to stretch and allow blood to flow in, causing an erection. However, there is another enzyme called PDE5 that breaks down cGMP and makes the erection disappear. If Viagra is used, it will strongly and effectively inhibit PDE5, making the cGMP level rise and maintaining a good erection. The patient should seek the advice of a medical professional before using Viagra. Basically, Viagra is not recommended for people with normal penile erections. Because Viagra will not make the normal person’s sexual ability better and better, not to mention disease-free and strong; on the contrary, some side effects may occur after taking it. For people with penile erectile dysfunction, a complete medical history and a thorough physical examination should be explored first. To determine the potential causes of the problem and to determine the appropriate treatment. Viagra pills are available in three different doses of 25 mg, 50 mg and 100 mg, with the same appearance and color, only the size is different, the following provides reference and recommendations for choosing the dose: 1. 25 mg is recommended for patients with any of the following conditions: (1) patients with moderate or poor liver function; (2) patients older than 65 years old; (3) patients taking erythromycin or anti-mycotic (3) Patients who are taking erythromycin or antimycotic drugs. 2. Patients with all of the following conditions may receive 50 mg and then adjust the dose according to the response: (1) Patients younger than 65 years of age; (2) Patients not taking erythromycin or antimycotic drugs. (3) Patients with normal heart, liver, kidney and vascular function; 3. The use of Viagra is generally not recommended for patients with any of the following conditions. (1) Patients who have had a myocardial infarction, stroke or life-threatening arrhythmia within the past six months; (2) Patients with heart failure or unstable angina due to coronary artery disease; (3) Resting blood pressure that is too low (blood pressure below 90/50mmHg) or too high (blood pressure above 170/100mmHg) (4) Patients with retinitis pigmentosa. Generally speaking, the higher the dose, the better the effect, but in turn, the greater the side effects. Therefore, it is recommended that the maximum dose of 100 mg per dose, exceeding it may be dangerous. Fourth, the use of Viagra fashion must follow the following recommendations (1) take a maximum of once a day; (2) take 40 to 60 minutes before the sexual act, and with sexual stimulation in order to maximize the effectiveness; (3) taken on an empty stomach or 2 hours after the meal to take the absorption effect is better (high-fat food will inhibit the absorption of Viagra, should be avoided); (4) do not and other drugs to help the penis erection together to prevent the occurrence of persistent erection of the penis. Of course, everyone’s situation is not the same, so patients with sexual dysfunction should go to the hospital and use the medication according to medical advice, so that they can receive good results.