The prefix Dr. in English refers to both medical doctors, and PhD graduates. What are the correct titles for these in Chinese? I believe that 医生 is correct to use as a title for a medical doctor, but I don't know if it can be used for a PhD.
All the information is taken straight from Wiktionary's English entries and the Chinese/Mandarin translations thereof. You don't really need to know any Chinese to find the information:
Doctor (physician): 医生, 大夫.
Doctor (person with a doctorate): 博士
Doctor (vet): 兽医
Doctor of Philosophy: 哲学博士
Graduate (from a university): 毕业生
博士 can be used as a title. For example you'd call me 焦博士 if I was an academic doctor (I wish).
毕业生 is usually used without a name attached; however, if you have to use it as a title (which is really unusual in either English or Chinese), then you can append the name of the person at the end of it: 毕业生James or 毕业生小焦.