Quinine may create Tryptophan deficiency

Unlike yeast, humans cannot make their own tryptophan and thus require dietary tryptophan, which is prevalent in meat. If quinine severely reduces tryptophan uptake, then it follows that people with preexisting tryptophan deficiencies would be especially at risk to this drug. The authors also note that tryptophan is important as a precursor for the brain chemical serotonin, so the enhanced tryptophan deficiency induced by quinine could explain why many of quinine's side effects are localized to the head region. They also note that side-effects could be averted simply by taking dietary tryptophan supplements in conjunction with quinine treatments. Journal of Biological Chemistry, 2009; 284 (27)

Bonnie - for anyone taking quinine who does not eat animal protein, a 5-HTP supplement is crucial and may be needed even for meat eaters.