Extra-Strength Super Power Juice is a strange drink that first appeared in Captain Underpants and the Invasion of the Incredibly Naughty Cafeteria Ladies from Outer Space (and the Subsequent Assault of the Equally Evil Lunchroom Zombie Nerds).
In the book, it was one of the strange juices found in Zorx, Klax, and Jennifer's space ship. It was found in the refrigerator by Captain Underpants, but was quickly taken by George because of Captain Underpants' habit of getting into trouble. When Zorx, Klax, and Jennifer told the trio to get into the jail cell, it was secretly taken by the heroes along with the Anti-Evil-Zombie-Nerd Juice and the Ultra Nasty Self-Destruct Juice. After the trio of heroes escaped the space ship and destroyed it, Captain Underpants fell into the deadly jaws of the Dandelion of Doom. Realizing their friend's (and principal's) fate, George and Harold reluctantly poured the drink into his mouth, giving Captain Underpants superpowers. After Captain Underpants killed the ferocious plant, it is likely that it was put into George's treehouse, since this was the next place it was seen.
It made appearances in the comics that were supposed to inform the reader of what had happened earlier.
In Captain Underpants and the Wrath of the Wicked Wedgie Woman, it was seen in George's tree house during the duo's struggle against Wedgie Woman. When she fell down and hit the wall, it spilled onto her beehive hairdo, giving it the ability to turn into multiple arms of hair. Using this new ability, Wedgie Woman gave George and Harold wedgies and taking them hostage.
The juice later appeared in Captain Underpants and the Preposterous Plight of the Purple Potty People. It was one of several items George took out of his backpack in an attempt to find the comics him and Harold made. If the reader looks closely at the picture, it is seen that George and Harold's grandparents are drinking the rest of the juice.
The Extra-Strength Super Power juice is described to be green and glowing, probably due to the fact that it is from space. It has effects that vary. It gave Captain Underpants the ability to fly, it gave Wedgie Woman's hair the ability to turn into arms of hair, and gave George and Harold's grandparents the ability to turn into Boxer Boy and Great-Granny Girdle.
In Captain Underpants and the Tyrannical Retaliation of the Turbo Toilet 2000, it is likely Melvin Sneedly replicated the Juice, or at least the effects of it.