There are 2 instances in the main arc which resemble the plot element you listed, one is when Itachi returns to Konoha where he tries to place Jiraiya under Genjutsu before trying to corner naruto alone but I am sure that's not the one you're looking for.
The second one the more close solution is in "Bringing Tsunade Back Arc" where naruto and Jiraiya try to convince tsunade to return to konoha to heal Rock Lee and Sasuke (who eventually got hurt by Itachi in the earlier encounter i mentioned). Tsunade drugs Jiraiya before trying to confront orochimaru to kill him its because she didn't want jiraiya to involve in the fight. I think you might be aware that Jiraiya, Orochimaru, Tsunade were called the Sannin *(Legendary three*) because they were a team once.
The drug causes jiraiya not to mold chakra properly causing him to fight at half his strength and he still summons Gamabunta (Chief toad) in that fight. I think that's the episode you're looking for.