Viagra is the common name for sildenafil, and taking sildenafil once may cause some side effects, but not too many. Sildenafil enhances the nitric oxide (NO)-cGMP pathway by selectively inhibiting PDE5, elevating cGMP levels and leading to relaxation of penile cavernous smooth muscle, which causes erectile dysfunction patients to have a natural erectile response to sexual stimulation. Because sildenafil has a vasodilating effect on the body’s circulation, some patients may experience hypotension, headache, dyspepsia, diarrhea, and rash after taking sildenafil. When the medication is used in accordance with the doctor’s instructions, the probability of experiencing these side effects is not too high. Sildenafil is contraindicated in patients who are using drugs containing organic nitrates and in patients who are allergic to sildenafil, as well as in patients with cardiovascular diseases such as heart disease and hypertension, and in those with glaucoma or elevated intraocular pressure. It is recommended that patients ask their doctor before use and use it under the doctor’s guidance.