Xiao Yao San vs. Chai Hu Shu Gan San

Both formulas are led by chai hu and both course constrained liver qi. The difference is temperament — one harmonizes while it moves; the other just moves.

The one-line difference

Xiao Yao San moves qi while replenishing — blood-nourishing and spleen-strengthening herbs make up most of the prescription. Chai Hu Shu Gan San (柴胡疏肝散, "Bupleurum Powder to Spread the Liver") is a purer, sharper qi-mover: chai hu backed by chen pi, zhi qiao, xiang fu, and chuan xiong, herbs whose whole job is to break through stagnation^[1].

When each fits

Chai Hu Shu Gan San's picture is acute and physical: hypochondriac and flank pain, distension, a sense of oppression in the chest — constraint expressed as discomfort in the body, in someone not notably depleted.

Xiao Yao San's picture is chronic and mixed: the same constraint, but in a person also running low — fatigue, poor appetite, menstrual irregularity — where a purely dispersing formula would move qi the patient cannot spare^[1].

The practical heuristic

The commentaries' shorthand: Chai Hu Shu Gan San for the robust patient with painful stagnation, Xiao Yao San for the depleted patient with smoldering stagnation. Duration matters too — dispersing formulas are short-course tools, while Xiao Yao San's build allows the longer courses that chronic patterns need.

Differential logic, not a recommendation — pattern diagnosis is a practitioner's call.