The fundamental concept of science-based medicine (SBM) is that medical practice should be based upon the best available science. This may seem obvious, but there are many important details to its application, such as the relationship between clinical and basic science. Clinical claims require clinical evidence, but clinical evidence can be tricky and is often preliminary. It is therefore helpful (I would say essential) to view the clinical evidence in light of all of the rest of science.
A thorough basic and clinical science analysis of a medical claim can be summarized by the term “plausibility,” or “prior probability” if you want to put it into statistical terms. When we say a certain belief is plausible we mean it is consistent with what we know from the rest of science. In other words, because of the many weaknesses of clinical evidence, in order for a therapy to be generally accepted as part of SBM it should have a certain minimal supporting clinical evidence and overall scientific plausibility.
These can exist in different proportions – for example one therapy may be highly plausible (it would be shocking if it were not true) and have modest supporting clinical evidence, while another may have unknown plausibility but with solid clinical evidence of efficacy. But no therapy should have clinical evidence that suggests lack of efficacy, nor extreme implausibility (not just an unknown mechanism, but no possible mechanism).