OPM has an FAQ page on prescription drug coverage which states that
Q. Are generic drugs likely to cause more side effects than brand name drugs?
A. There is no evidence of this. The FDA monitors reports of adverse drug reactions and has found no difference in the rates between generic and brand name drugs.However, the Washington Post reports today in its health section about a Medco survey finding that 27% of physicians are of the opinion that generic drugs are more likely to produce unwanted side effects than brand name drugs (in contrast to 5% of pharmacists and 10% of consumers).