You may provide any information that is true, that you have a good faith belief is true, or that represents your honest and supportable opinion. You should not provide any medical information that you may have on file.
There is a persistant and totally mistaken belief that employers are prohibited from giving any information other than dates of employment, job title, and (depending on who you hear it from) possibly salary history. There is also a persistant, and just as mistaken belief, that employers may not give out any negative information that might prevent the employee from getting the job. There is no truth whatsoever to either of these beliefs. In fact, there are some situations where an employer may find himself in legal trouble if he does NOT provide certain negative information that he may have in the employee's record. You may not lie, and you may not be deliberately misleading. But no law anywhere prohibits an employer from telling the truth.