How much sildenafil is best to take depends on the individual, and the general recommended dose is 50mg; it does not have to be taken at noon, and is usually taken as needed 1 hour before sexual activity. Sildenafil is a cyclic guanosine monophosphate-specific phosphodiesterase 5 inhibitor that promotes penile erection in response to sexual stimulation. It is used clinically to treat erectile dysfunction. The best amount of sildenafil to take depends on the individual, the general recommended dose of 50mg, based on the individual use of the effect and tolerance of the drug, can be increased to a maximum of 100mg, or reduced to 25mg. up to once a day, do not necessarily have to take at noon, about 1 hour before sexual activity can be taken as needed. Sildenafil may cause adverse reactions such as headache, flushing, indigestion, abnormal vision, nasal congestion, back pain, and myalgia. Sildenafil is contraindicated for use in people with a history of allergy to this drug, and should not be combined with nitrate drugs such as isosorbide nitrate, or guanylate cyclase such as liothyronine, as this may cause low blood pressure. It should be used under the guidance of a doctor, do not increase the dose or the number of times the drug is taken to avoid discomfort. Seek medical advice immediately if discomfort occurs.