It depends on what you are doing. I know the Army Reserve has a program where they pay you during residency and then you owe them time afterward. With something like that you would not be MOS (military occupational specialty) qualified (and therefore, technically nondeployable) until you finish your residency. With this type of thing you pick your specialty and apply for a civilian residency, then sign up once you get a residency spot. There are programs that pay for medical school, then you may or may not have to do a military residency, and they may or may not be able to influence your specialty choice (you would have to talk to people who have done it for more details). Again, you would not really be deployable until you are done with residency (although, I guess they could send you after intern year as a general medical officer if they decided they wanted to). If you are not in one of those programs, and they are going to be training you in a military specialty, you would definitely be deployable from the moment you complete training. I found this out while in med school. Despite the fact that I had only a year left to finish, they deployed me for a year and a half as a nurse (my specialty when I joined). Your unit may try to work with you, but in the end it only matters what the needs of the service are for the final decision on whether to deploy you. When I applied last year (prior to the end of my enlistment contract) I was asked about my deployability status at each of my interviews. I don't know how much of a role that played in my not matching (there were other factors for sure). If you are definitely protected from deployment during residency (don't just take someone's word for it, if it isn't in your contract it doesn't exist-a good thing to keep in mind for everything they promise you at the recruiter's office), I don't think it would be a disadvantage, but if you are in a critical wartime specialty with the potential for deployment, I think some program director's would prefer not to have to deal with that.