Athenahealth Inc., a Watertown company that provides electronic record and billing systems to hospitals and other medical industry clients, said it has agreed to buy Epocrates Inc., which has developed point-of-care medical applications that about 330,000 doctors use as reference tools, in many cases on their iPhones.

For example, one Epocrates app is a drug interaction tool that allows a doctor to check whether a medication they plan to prescribe interferes with a medication that their patient is already taking, Athenahealth said.