medical and drug trials routinely have severe p-hacking issues. And there have been a lot of reproducibility problems reported with, e.g. preclinical cancer research, often lacking slam dunk evidence.
Due to my medical problems I have been reading medical literature for 25 years, and indeed it is a catastrophe of p-hacking and the like, incompetent statistical analysis, ven very often there is a basic misunderstanding of what p-values mean. You routinely see researchers claiming “no effect” when the p value is slightly over 0.05.
Usually, medical papers are misleading in some serious way. The best you can hope for is that they waste the vast majority of the value in the data.
People who read abstracts only and thing they are learning something are deluding themselves. You can to go through the methods section carefully and even then not all the shenanigans are disclosed, and look very closely at sponsorship of the parties to the study (researchers, journal editors, institutions etc) to pick up the extreme biases that result from sponsorship.
Having been on a “close to vegan” diet for many years with quite catastrophic results I would like to point out a couple of subtleties in this sphere.
The deficiencies associated with any diet can take years to appear. For example in a zero B12 diet it can take 6 years for a deficiency to appear. During this time the body is in effect “eating itself” i.e. sunstituting one form of animal food (oneself) for food eaten.
Often, as in my case, the initial switch in diet was associated with an improved sense of well-being due to not eating junk food any more. This is sometimes referred to as the vegan honeymoon.
There is often a large gap between the level of supply of nutrients that will avert frank deficiency and an optimal supply. But it is often assumed for example that if you don’t have scurvy that you have plenty of vitamin C.
I learned the hard way that much of the scientific literature is worthless or worse (anti-knowledge). In particular you cannot learn anything reliable from reading only abstracts. This may be obvious to some but is not obvious to many. Financial and ideological biases loom large and a lot of research is only published because of the “publish or perish” mandate. Purely observational studies are almost all useless, and “corrections for confounders” are not reliable.