A dispensing cap sealingly mountable within the throat of a beverage container has an open-bottomed flow tube connected to a surrounding skirt by a resilient diaphragm, forming a reservoir surrounding the flow tube. A membrane attached to the periphery of the skirt seals off the bottom of the reservoir. The lower end of the flow tube extends into the reservoir and is adapted for piercing the membrane. A downward force applied to the flow tube will resiliently deform the diaphragm, causing the lower end of the flow tube to pierce the membrane, thus allowing the contents of the reservoir to flow into the container. The flow tube extends above the diaphragm and terminates in a flow assembly operable between a closed position and a “consume” position in which the contents of the container can flow through the flow tube and out an opening in the cap sleeve.