There are many reasons why tadalafil works, but not twice, such as taking the wrong dosage of the drug, the wrong way to take the drug, and so on, the specific reasons need to be analyzed. 1. The dose of the drug is not right: tadalafil belongs to the prescription drugs, usually the doctor according to the patient’s own situation to choose the right dose, if the beginning of taking effective and then ineffective, you need to timely follow-up, adjust the dose of the drug, follow the doctor’s instructions. 2. Taking the wrong way: tadalafil needs to be swallowed whole when taking, not broken to take. Patients need to take half an hour before sex, and need to receive a certain amount of sexual stimulation in order to produce a good erection. Tadalafil may cause adverse reactions such as headache, indigestion, back pain, etc. It should be noted that it is contraindicated for those who are allergic to the drug. The drug needs to be used under the guidance of a doctor, do not blindly use the drug on your own, so as not to cause adverse consequences.