Quite simple... generics are not absolutely 100% the same...
They have also been able to take advantage of a me-too environment where a company develops a product after years of torturous research and testing, when it can effectively be copied by a professional chemist with the gas chromatograph and other assorted testing equipment.
Once a product is run thru the US FDA, its own particular DNA is open to the world, for validation, but more importantly for the copying....
Viagra is the brand name... sildenafil citrate is its chemical name... you can buy the name brand from Pfizer, or you can take one of the many "knock offs" that exist as substitutes.
But they arent exactly the same... you will have different side effects, different reactions to other drugs, or foods or what have you in any generic alongside the brand name.
Thats not to say they are bad, but in some cases, the side effects arent worth the effort...
The likeness is you can buy Coke or you can buy Australia's Choice Cola... same brown bubbly stuff in a can, but they arent exactly identical...
