A plastic jug filled with milk; keep upright and refrigerated at all times.
While milk is an excellent source of Calcium, it is a rare delicacy only available through Resistance and Black Market groups operating in the Outlands due to the loss of the vast majority of farms and domesticated cows on Earth. Due to the low quality (or complete lack) of refrigeration available to Citizens, it often spoils before it can be enjoyed.
To use a Milk Jug, either click on it in your inventory and choose the "Use" context button OR look at a dropped Milk Jug, press "E", and choose the "Use" context button. This will make your character drink the entire jug of milk.