A gastroretentive, sustained-release dosage form including 5-hydroxytryptophan (5-HTP) as an active ingredient and low-dose carbidopa is described. For example, the dosage form can be provided as a bilayer tablet comprising a swelling layer and a modified release layer, where the 5-HTP and carbidopa are both included in the modified release layer. The dosage form provides for essentially parallel release of the 5-HTP and the carbidopa with, for instance, release of 80% of the 5-HTP and carbidopa at about 5 hours to about 12 hours.