It can be dangerous to take sildenafil if you also take medicines called nitrates often given for chest pain. Take sildenafil as needed before sexual activity. Without physical action to the penis, such as that occurring during sexual intercourse, sildenafil will not work to cause an erection. Both Cialis and Viagra work quickly. It works for up to 36 hours.CVS Health. Pharmacy; Store Locator; Shop; Photo; Corporate Information; Careers; test1.ru (opens new tab) HealthHub; Optical; For our commitment to high-quality health care, we’ve earned The Gold Seal of Approval® from the Joint Commission (opens new tab) We’re a US Department of Veteran’s Affairs Authorized Retail Clinic. Read reviews and enjoy FREE shipping on most orders when you shop for sexual health products online at CVS. Browse top brand sex products today! This medicine is available on prescription and you can also buy it from most pharmacies. It comes as standard tablets, chewable tablets, and as a liquid that you swallow. Key facts. It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex.