The way the drug industry works is that when drugs are new, the drug company that developed them has a patent, that means that no one else can copy them and sell them. This is fair enough, the company that made them has had to spend loads of money developing the drug, licensing it and marketing it; they need to recoup this back. But what happens when the patent runs out? Well, this means that other companies can make this product as what we call a clone. That means that as long as every aspect of it is the same as the original a marketing license can be granted to another company. Still with us? Let's take an example
Frontline for dogs and cats made by Merial is a case in point, the ingredient fipronil has been around a while and is a great product, brand leader one could say. But now there are others available like Fipnil which are exactly the same but cheaper.
We are not saying there is anything wrong with Frontline but lets face it there is nothing wrong with Fipnil either. Hope this helps.