Kelp Shakes are an addictive and potentially toxic soft drink served in the Kelpshake restaurant chain. They only appear in the episode "Best Frenemies."
The drink comes in a light green-colored bottle with a light yellow lid and light yellow straw, and on the bottle is a light yellow label that reads the drink's name in light red text. The color of the drink itself is most likely the same color as the bottle it comes in.
The drink was very popular and tasty, as Plankton described it as having "a pungent aftertaste." However, Mr. Krabs described his first sip of it as tasting like a "wet gym sock" before the flavor kicked in. It is light green and the bottle contains a chartreuse straw. At one point it was so popular that restaurants were on each block of Bikini Bottom.
It is eventually proven by Karen that the drink is 99% Kelp Juice and the remaining percentile is an unknown radioactive material that, at the end of the episode, is revealed to make the consumer grow green hair on their entire body as well as grow abnormal round buttocks.
At the end of "Best Frenemies," SpongeBob, Mr. Krabs, and Plankton are the only ones seen being the victims of the Kelp Shake. However, since almost everybody else in town drank a Kelp Shake, they all suffered the same symptoms.
The body-changing side effects of the beverage cover the entire body in fuzzy kelp similar to a gorilla's fur and their clothes vanish as the fuzzy kelp grows all over their bodies. For some odd reason, however, it takes everybody except for Mr. Krabs and Plankton about two days to grow green hair and round buttocks.
Because of these symptoms, the authorities ordered the complete shutdown of all the Kelpshake restaurants and proceeded to dispose of the dangerous chemicals used to produce them. While everyone eventually returned to normal, the Kelp Shake itself was never sold anywhere again.