Part of a statement at the annual meeting of the Massachusetts Medical Society (30 May 1860), generally quoted in a simplified form omitting Holmes's exceptions including opium and anaesthetics.
Throw out opium, which the Creator himself seems to prescribe, for we often see the scarlet poppy growing in the cornfields, as if it were foreseen that wherever there is hunger to be fed there must also be a pain to be soothed; throw out a few specifics which our art did not discover, and it is hardly needed to apply; throw out wine, which is a food, and the vapors which produce the miracle of anaesthesia, and I firmly believe that if the whole materia medica [medical drugs], as now used, could be sunk to the bottom of the sea, it would be all the better for mankind,—and all the worse for the fishes.
As quoted in a review of Currents and Counter-currents in Medical Science (1860) in The American Journal of the Medical Sciences, Vol. 40 (1860), p. 467 http://books.google.co.uk/books?id=zHdDAAAAYAAJ&pg=PA467
Paraphrased variant: If all the medicine in the world were thrown into the sea, it would be bad for the fish and good for humanity.