When oral medications might not be safe Check with your health care team before you take any medicine for erectile dysfunction. The medication was developed by U. Do not mix the liquid with other medications or add anything to flavor the medication. Tadalafil Cialis. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. There were some reports of headaches and nausea, however.The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. For more information, you can refer to Viagra’s prescribing information. If you develop serious side effects while taking Viagra, call your doctor right away. If the side effects seem life. It depends on a few factors, including your age and why you’re taking it, since Viagra is not only used to treat ED. Your doctor will tell you how much you should take. For help with erections.