Kelp Shakes were an addictive and potentially toxic soft drink served in the Kelpshake restaurant chain. They make their first and only appearance in the episode, "Best Frenemies."
The drink was very popular and tasty (although Plankton described it as having "a pungent aftertaste"). The drink bottle also contained a straw.
It was eventually proven by Karen that the drink was 99% kelp juice and the remaining percentile was an unknown radioactive material that, at the end of the episode, was revealed to make the consumer grow green hair on their entire body, and abnormal round butts.
In the end of "Best Frenemies," SpongeBob, Mr. Krabs, and Plankton were the only ones seen being the victims of the Kelp Shake. However, since almost everybody else in town drank a Kelpshake, they all suffered the same symptoms.
The body changing of the Kelp Shake made the consumer look similar to a gorilla, and even your clothes, socks, underwear, and shoes became green kelp too. For some odd reason, however, it took everybody except for Mr. Krabs and Plankton about two days to grow green hair and round buttocks.
It is possible that the Kelp Shakes were all made at the same time, and were going to mutate the characters at that exact time, regardless of when or how much people drank as Mr. Krabs and Plankton only had one bottle between them while others had many more.