It would depend upon the drop down menu. If you mean a select element, there is no support for onmouseover of the options in IE 6 or 7. The specification calls for one though, so in FF (probably others) you can do it. But for the widest compatibility, just give your options a title attribute. This will work in all but IE 6:
If, on the other hand you have a menu script that contains elements like divisions and links, you could have a tool tip scripted onmouseover event for those items.