“ Amitriptyline caused decreased concentrations in rat plasma of the amino acids tyrosine and tryptophan, which are … to monoaminergic neurotransmitters, and also of those other large neutral amino acids (LNAAs) (valine, phenylalanine, leucine and isoleucine) with which tyrosine and tryptophan compete for … carrier‐mediated transport from plasma into the brain.”