Good question. I assume you are referring to having a stomatch for the blood and gross stuff you see as a doctor. I have known a few people who had pretty significant phobias of blood and guts, and they managed to find a way to get through it. Most people have some sense before they even get to medical school that they are squeemish about blood. You can get a decent amount of exposure from looking through some of the medical text books that have pictures of surgeries, and watching videos would be an even better way. If you volunteer on a hospital floor, you can get some limited exposure, but it isn't the same as what you will experience when helping out with surgeries. It's not uncommon for people to have a lot of anxiety about doing the dissections in gross anatomy as well.

I commented on the topic in this thread on blood and guts and so my comments there may be helpful as well.

Most people certainly adapt, and those with real phobias / fears can adapt as well if they are motivated. In the end, I think most doctors find the emotional and physical demands much more stressful than the exposure to blood and guts.

If you are talking about having the stomach for the emotional and physical demands - well, that is something that you probably won't know for sure until you are there. I have to admit that nothing before medical school prepared me for the intensity of the clinical rotations and residency experiences.