John’s wort. Continue to take sildenafil even if you feel well. You should only be treated with one of these products at a time. Medicines that block an enzyme called CYP3A4. To provide you with the most relevant and helpful information, and understand which information is beneficial, we may combine your email and site usage information with other information we have about you. If you experience chest pain, dizziness, or nausea during sexual activity, call your doctor immediately and avoid sexual activity until your doctor tells you otherwise.Viagra comes as tablets you take by mouth. You’ll only need to take it as needed, before having sex. You don’t need to take Viagra regularly each day. Viagra . The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in .