It's been reported that most online pharmacies are not legitimate, and often sell fake drugs. For this reason, one must be extra careful when filling prescriptions from online vendors. Make sure the pharmacy is real first of all and then do your due diligence to make sure it is selling real drugs. However, some online pharmacies are legitimate. NetMeds even offers coupons for their huge selection of medications available in India where brick and mortar pharmacies may be few and far between. Mail order pharmacies are a great way for developing countries to distribute medications to residents, and I would expect to see more of these popping up all over the world.
Many of the fraud and fake pharmacies are directed at people in the United States. The lure of low prices in Canada often drives consumers to those mail order options outside the country, but one must be aware of the dangers. The FDA has a great article that provides useful pointers to anyone thinking about ordering online - Know Your Online Pharmacy. Additionally, there are a few common sense things to keep in mind.
Do a Google Search
Usually, scammers are that well funded, and their phony pharmacies often haven't built legitimacy around the web. For example, they may not have a decent looking website, and you are likely to encounter bad reviews. I don't believe there is a better incentive for negative feedback than for someone to be ripped off, and you're likely to encounter that bad publicity if you do a little poking around.
Age of the Company
This is not always a good tell because some of these are legitimate start-ups that haven't been around long. However, one that has been around for years is probably less likely to be a scam that one that is only a few months old. It's amazing what you can find out about a company this way from the name of the CEOs to the number of employees. If it's registered on some type of stock exchange, then chances are better.
Order From Known Companies
Many established pharmacies in the United States and elsewhere already offer mail-order options, especially for prescription refills. They'll mail it right to your door, and since it's a company you already know and trust, you won't have to worry. This somewhat negates the incentive to order online because the price probably won't be any better that it is in the store. Here is a good rule of thumb: if the price is too good to be true, it probably is.