That is why you have the FDA - to make sure drug companies test their products properly.
FDA regulation doesn't encourage manufacturers to look at long term effects.
EDIT: I'm wrong about this. Thanks hga.
In granting the initial approval. There's various stuff to watch for bad long term effects, which it must be noted also take a long time to show, and are also known to be tied to uniquely vulnerable sub-populations who won't necessarily be well represented in clinical trials.
Heck, for some time I took a drug what almost certainly would have never gotten approval if anyone had realized it killed the livers of a small number of people taking it, when the total number was large enough. The FDA black boxed it and the original company stopped manufacturing it, but many like me who'd passed the danger period and had no signs of liver problems continued taking a generic version for a while.
And how many drugs have been pulled off the market after additional testing hasn't been completed?
See GFK_of_xmaspast's link to the FDA page on Vioxx, where the drug company's monitoring effort pulled the plug before anything else.
I thought that they had been the instigators in vioxx, but not quite: http://www.fda.gov/Drugs/DrugSafety/PostmarketDrugSafetyInfo...