Generally Viagra will cost several dollars less if your prescription is filled at a walk-up pharmacy. However, many consumers appreciate the convenience and privacy of purchasing online and having the product discreetly delivered. Legitimate online pharmacies ask you to fill out an efficient health questionnaire, and many offer a free online consultation, so this saves you a visit to your doctor's office and pharmacy -- and the associated time and expenses. 

Never purchase medications from an online pharmacy that does not require you to fill out a medical questionnaire that is reviewed by a doctor. Prescription medications should be used in consultation with, and under the supervision of, a physician who is familiar with your medical history, current medications, etc. Otherwise, you could be putting your health at risk.