Are online pharmacies safe?

Are online pharmacies safe? I mean really safe? I have often thought that they would be a great way for us to save some money and have found a few sites online that sell the medication that my boyfriend takes. But I am not sure if I can trust them. I saw a thing on dateline once where places like this were selling fake medications. I would hate to see one of these pharmacies be like that. I just can not understand how someone could intentionally do this? Why would you sell a person meds if you knew they were unsafe or were not doing the job that they are suppose to do?
So for know I guess I will just have to suck it up and pay the full price for the medications and know that they are safe and will do the job they are suppose to do.