According to the editor of the British Medical Journal, Richard Smith, only about 15% of medical interventions are supported by solid scientific evidence.
According to Professor David Eddy, of Duke University, only 1% of the articles in medical journals are scientifically sound and many treatments have never been assessed at all.
The British Medical Journal (BMJ) website Clinical Evidence reports that, of the 2404 treatments they have surveyed, only 15% are rated as beneficial, while 47% are of unknown effectiveness.
Where is the wisdom…? The Poverty of Medical Evidence
By RICHARD SMITH, Editor of BMJ
British Medical Journal 1991 (Oct 5); 303: 798–799