Use the oral syringe provided with your medication to measure and take your dose. Cialis and Viagra are equally effective. Medicines that you take by mouth are called oral medicines. What side effects can this medication cause? Sildenafil usually should not be taken more than once every 24 hours. Kidney disease that needs to be treated with dialysis.These medicines all work well for ED. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the . 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.