My Gym Partner's a Monkey (2005)
S1E1 Inoculation Day
The students are apprehensive about receiving their vaccinations, prompting Nurse Gazelle to resort to a blowpipe for the injections. In a mishap, she accidentally administers Jake's shot to Adam. Initially, Adam appears unaffected, but soon after, he starts to regress, behaving like a simple-minded ape.