That's interesting, but I was talking about the success rate of someone with a terminal illness going the clinical trial route. Sorry, I now see that my question was not so precise.

For cancer, it doesn't seem to impact survival odds at all [1]. In other fields it may improve metrics a small bit but that's largely because in clinical trial patient selection, they're very careful to exclude anyone with an even remotely confounding factor (like weight/BMI).

[1] https://www.science.org/content/article/joining-cancer-trial...