There is a pharmacological concept called "delayed drug toxicity".

It is complex, but exists for many drug entities. For example, the tendon ruptures from fluoroquinolones can happen up to a year or more after stopping the drug. It took over a DECADE to figure this one out.

Drugs that damage cells/DNA or that set off an autoimmune cascade, are actually exhibiting delayed toxicity.

You can Google "delayed drug toxicity" for more information.
Suffice it to say, delayed toxicity typically appears post marketing and does not show up in many drug studies that the FDA initially requires. Several fluoroquinolones have been taken off the market for this reason. There were at one time many more than we have available now.

Here is an explanation of delayed allergic reactions that occur long after the suspected trigger drug is supposedly gone from tissues: serum sickness is possible with some antibiotics:
http://www.nlm.nih.gov/medlineplus/e...cle/000820.htm

Steroids exhibit also a trait, after being taken, they can trigger bone necrosis which typically attacks the hip, which can fester for months/years leading to destruction of the joint. This is a delayed toxic event also.