Viagra and Viagra are both trade names for sildenafil; cialis is a trade name for tadalafil, all are drugs for treating male erectile dysfunction, with their own characteristics, and it is not possible to simply compare which one is better. 1. Sildenafil: Sildenafil has a short onset of action after taking it, usually about 30 minutes, and erection hardness is a bit higher, but the duration is shorter. As food may affect the effect of the drug, generally need to be used on an empty stomach. 2. Tadalafil: Tadalafil belongs to the long-acting drugs, the drug is not affected by food, the drug effect lasts a long time. The hardness of erection is not as good as sildenafil, but the chances of side effects such as dizziness and headache are less. This class of drugs is contraindicated in people who are allergic to any of the components in the product, patients who are taking any form of nitrates, guanylate cyclase stimulators, women, children and other people. In general, sildenafil and tadalafil are prescription drugs, taking the drug may appear dizziness, headache, vomiting and other discomforts, if you suffer from erectile dysfunction of the population, it is recommended to go to the hospital in time, under the guidance of the doctor to choose the medication, do not blindly use the drug.