I'm in pharm school right now and this a topic that my group and I had to research on for a class project. Anyways generally it's usually a very bad idea. The most legit online pharmacies are VIPPS (Verified Internet Pharmacy Practice Sites) certified by the NABP (National Association of Boards of Pharmacy). And those sites as already mentioned require medical records, prescriptions, etc from your doctor. And in the end, the price comes up just about the same.
Online pharmacies other than VIPPS certified you should be very aware of because often times, the medications come from foreign countries, are mislabeled, expired, not approved by the FDA, mishandled, etc. Sometimes it is very hard to tell between a legit and a fake medication even when placed side by side.
Do not trust word of mouth either. That is probably the most dangerous thing regarding this topic. What happens is people who claim they have a good experience with a particular online pharmacy are not medical professionals and base their claims solely on their experiences. Many of these people aren't sure if the benefit they're getting from drug is the maximum benefit they should be getting.
Don't be tempted by price because this is one of the medications that you should definitely have a doctor prescribe you. For one thing, the dosage is calculated by weight. And depending on your doctor, he or she knows how to adjust the dosage accordingly. Sure you can find the formula online but you're putting your health at risk simply because you have no idea what you are doing. There's a reason why it's so heavily regulated. You need to come in for mandatory blood tests once a month to make sure everything is within range of normal limits.
If you go to a university, you are probably enrolled in a mandatory health plan that can cover a large part of the costs of checkups, bloodtests, and medications.
If you don't heed my advice then I wish you the best of luck.