Gut Milk is a fictional vanilla-blast flavored supplement and multi-level marketing product featured in the TV show "Only Murders in the Building". It is a probiotic-rich drink that is marketed as a beverage and a business. According to a Reddit user, it is a multi-level marketing product that has probiotics for digestive health and looks milky. The shows merchandise includes a graphic tee and a stemless wine tumbler inspired by the drink. However, in reality, there is no such thing as gut milk. It is a fictional product created for the show.