Evidence-based medicine appears to have taken over, but 'evidence' is not all it seems.
In 'Bad Pharma' (everyone should read this), Ben Goldacre, psychiatrist, epidemiologist and journalist, sums up his book in a single paragraph: 'Drugs are tested by people who manufacture them, in poorly designed trials, on hopelessly small numbers of weird, unrepresentative patients, and analysed using techniques which are flawed by design, in such a way that they exaggerate the benefits of treatments. Unsurprisingly, these trials tend to produce results that favour the manufacturer. When trials throw up results that companies don't like, they are perfectly entitled to hide them from doctors and patients, so we only ever see a distorted picture of any drug's true effects. Regulators see most of the trial data, but only from early on in a drug's life, and even then they don't give this data to doctors or patients, or even to other parts of government. This distorted evidence is then communicated and applied in a distorted fashion. In their forty years of practice after leaving medical school, doctors hear about what works through ad hoc oral traditions, from sales reps, colleagues and journals. But those colleagues can be in the pay of the drug companies - often undisclosed - and the journals are too. And so are the patient groups. And finally, academic papers, which everyone thinks of as objective, are often covertly planned and written by people who work directly for the companies, without disclosure. Sometimes whole academic journals are even owned outright by one drug company. Aside from all this, for several of the most important and enduring problems in medicine, we have no idea what the best treatment is, because it's not in anyone's financial interest to conduct any trials at all. These are ongoing problems, and although people have claimed to fix many of them, for the most part they have failed; so all these problems persist, but worse than ever, because now people can pretend that everything is fine after all.'
Ben goes on to explain this in more detail, starting with the problem of missing data. Negative trials tend not to get published, so doctors are only aware of published trials that make a drug look good - this is bad science, and is not 'evidence' at all. This 'evidence' is then spun by drug companies and before you know it, doctors are sucked in - to the detriment of patients.
Sound clinical reasoning depends on understanding the nature of the evidence available to us - beware!