Postpartum blues is a healthy range of sadness that peaks on day 5 after giving birth in most women. Severe postpartum blues is a high-risk state for postpartum depression. A dietary supplement was designed to compensate for a temporary rise in a brain protein, monoamine oxidase A, which occurs on postpartum day 5. This study tested whether this dietary supplement reduces the sadness associated with the postpartum blues. Total levels of tryptophan and tyrosine in breast milk are not affected by this dietary supplement. Women received the dietary supplement over postpartum days 3–5 or received no supplement. This dietary supplement dramatically reduced the vulnerability to sadness on postpartum day 5, the peak of the postpartum blues, eliminating the prodrome of postpartum depression.