So why can't we approach psychaitry with the same 'evidence based medicine' paradigm that the rest of medicine uses? Why can't we trust that what Randomised Controlled Trials(RCT's) tell us?
The short answer is that the complexity of the brain in its capacity for varied behaviours is unimaginably greater than the rest of the body put together. I'll elaborate later or elsewhere, but essentially the behaviour of a muscle is largely dependent upon how many muscle fibres are contracting at a given moment, not the precise selection of which fibres are contracting: take ten random selections say 20,000 fibres, contract them and leave the rest alone, and the outcome is a similar amount of force. All that matters with a muscle is the amount of force extended at the tendons at each end. The brain is not like that.