Some 60-70 years ago already, Prof William Boyd wrote about a higher tendency of stomach bleeding in the O type blood group. One of the reasons why I would think twice before advising aspirin to patients with this blood type.
Dr Peter D’Adamo has done major research on blood types and nutrition, but he has been mostly ignored, even by alternative practitioners, just on theoretical grounds. I don’t know if all that D’Adamo states is proven, but doctors should look more into his books than just criticise him. Probably out of “jealousy” without ever trying some of his suggestions, especially concerning pulses [kidniot]