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 was very popular and tasty, although Plankton described it as having "a pungent aftertaste." It is light green and the bottle contains a chartreuse straw. It seems it is so popular, that restaurants are 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 make the consumer look similar to a gorilla and even the consumer's clothes, socks, underwear, and shoes turn into green kelp too. For some odd reason, however, it takes everybody except for Mr. Krabs and Plankton about two days to grow green hair and round buttocks.