Physiology particularly human physiology or medicine is a particularly difficult science in practice. A proper experiment particularly in human physiology is frequently either unethical, impractical or otherwise impossible. Proper evidence from experimentation and the scientific method is often unavailable. The void that this creates leads to a difficulty that confuses proper and legitimate understanding. There is great pressure to invent truth and call it evidence when truth can not be determined.
There is a struggle to know when scientific reasoning based on anatomy, physiology, biochemistry etc. will produce an expected result or when the complexity of the human body prevents such an approach. There are some spectacular failures of educated rationalization in medicine. The debacle known as VIOXX comes to mind. In the case of a failure of scientific reasoning, the problem is not just the complexity of the system that complicates the analysis. Bias and a conflict of interest are always an influence and may infiltrate the minds of those who formulate an approach. Declaring that a treatment is “evidence based” does not prevent the introduction of bias or downright misconduct.
“It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.”
― Mark Twain
"All you need in this life is ignorance and confidence; then success is sure."
― Mark Twain