The wormwood was primarily used to aid in the digestion of the potion but it also played a role in stabilising bacteria in the gut. While the bacteria did not cause depression it often added to the symptoms and general misery associated with depression. There was some muggle literature linking gut health to depression.  Felix suggested ginger as an alternative to wormwood. The muggle spice was not used much in potions but muggles often used it for nausea and it was considered to be safe for use during pregnancy. Severus rolled his eyes at his own short-sightedness, he himself had drunk gallons of ginger tea early in his pregnancy.  