Morphine in cow and human milk: could dietary morphine constitute a ligand for specific morphine (mu) receptors?

Morphine has been found in cow and human milk at concentrations of 200 to 500 nanograms per liter. Multistep purification yields a material that has immunological, biological, pharmacological, and chemical properties identical to those of morphine. Similar morphine-like material, which has been tentatively identified in some common plant sources, may be a ubiquitous dietary constituent and a possible source for the material in milk. Since morphine (mu) receptors have a low affinity for enkephalins, and since morphine-like materials have been described in brain and intestine, it is possible that morphine in food may be the source of this material and a normal ligand specific for mu receptors.