Medical ethics tends to prohibit it, especially in a clinical setting. It's seen as doing harm to the patients, which physicians tend to frown on.

I've even heard of some papers on fasting, retrospectives -- which I think aren't technically studies but an aggregation and analysis of patient data, collected from events that have already happened, which patients voluntarily undertook -- that were banned from journals because the results may have 'encouraged harmful behavior.' Don't have any sources on that and it's a memory from like 6-8 years ago though.