An erection usually occurs with sexual stimulation about 0.5 to 1 hour after using 20mg of tadalafil. Tadalafil is a prescription medication for the treatment of erectile dysfunction in men, after using 20mg of tadalafil usually about 0.5 to 1 hour, an erection will occur under sexual stimulation, after the use of the drug if there is no sexual stimulation usually will not be on their own erection. The first recommended dose of tadalafil is 10mg, and 20mg is only recommended if 10mg does not work well. the higher the concentration of the drug, the higher the chance of adverse reactions, so it is not recommended to use 20mg right from the start. Tadalafil may cause dizziness, headache, persistent penile erection, nausea and other uncomfortable symptoms after use. And it should not be used together with nitrates. It is also not recommended for people with liver or kidney insufficiency. Overall, tadalafil needs to be used under the guidance of a doctor and cannot be used blindly by yourself to avoid serious adverse reactions.