To qualify for the HPSP program, one first must be accepted to medical schools. In fact, you can’t apply unless you’ve already hold a MD or DO acceptance.
Once accepted, you can apply for the HPSP program, but admission to the program is competitive. Additionally, the applicant must meet all physical, medical and security background requirements for military enlistment.
I know several people who have attended medical school on HPSP scholarships. Three of D1’s medical classmates did, as did one of her co-residents. It’s a nice deal if you can get it. (But note: military doctors don’t get to choose where they’re stationed. Instead of Japan or San Diego, they could end up in Afghanistan, Iraq or patrolling the South China Sea.)