There have been numerous medical and scientific studies with N=1, known as N-of-1 trials. These trials are very useful in chronic conditions where symptoms are stable and measurable, allowing for multiple crossover periods to assess treatment effects accurately.
Also…don't forget all the medical discoveries based on self-experimentation:
Werner Forssmann performed the first heart catheterization on himself. He got the Nobel prize.
Barry Marshal injected himself with heliobacter pylori to prove that it causes stomach ulcers. Also got the Nobel prize.
Jessie Lazear allowed himself to be bitten by mosquitoes infected with yellow fever to prove his hypothesis that mosquitoes were the vector for transmission. No Nobel prize, but he did contract the disease, thus proving his hypothesis…before dying from yellow fever two weeks later.