The Growth elixir is a concoction made by Phineas and Ferb to make Baljeet's watermelon grow bigger for the Biggest Watermelon Contest at the Midsummer's Festival. While it was only meant to be used for the watermelon, Candace also decided use it to make herself two inches taller to become a candidate as the next Flawless Girl after finding that she was just about two inches too short to do so due to not being aware of it's full effect since she had watched Phineas and Ferb use it on Baljeet's watermelon, but when it only grew a few inches before it later took full effect, she just assumed that's all it did before leaving to find the booth and later took it from them to use without their knowledge or consent, it did work on her as she was able become tall enough to get into the running to be one. However, this would actually backfire on her as the elixir ended up working too well since it turn out to be much more powerful than what she thought and eventually made her grow 50 feet tall after her nose and her head grew when she was questioned by Stacy if there was something different about her before running into a tent to hide, right before the rest of her body grew to the size of her head and then growing again one more time to completely finish her gigantic growth.
Candace asks if the boys can undo it after sneaking over to them with the tent she was hiding in on her back and asks them to do something to reverse the effects it had on her before the Flawless Girl auditions began in the next couple of hours as she believed that they were the ones who did this to her despite being the one who used it on herself. Phineas mentioned that the elixir only works in one drop; putting more than one drop won't make the object or person grow past two inches before it takes full effect later on and makes the object or person grow gigantic. Candace didn't think she would actually get picked to be the next Flawless Girl because of her gigantic size after being completely effected by it as she didn't like what it did to her and thought it made her a freak instead, this actually ended up being the opposite as to what she intentionally thought as she was easily picked as a Flawless Girl later on because of her gigantic size since it made it so that she is able to heighten expectations to such an unreachable level so high that no other girl would ever be able to live up to it and for her to not be seeing that coming. But decided to reject that offer after changing her mind about wanting to be one and making beauty the focus of her life.
Later on, the elixir ends up in Doofenshmirtz's Smellinator after Blanca Dishon and P. P. Otter had a fight over it shortly after Candace left when she realized that the both of them take advantage of people's insecurities to make money whether it was to make them feel better or worse about themselves upon them getting into an argument over her along coming to a conclusion that whether a Flawless Girl or a freak, she knows full well that it's all still pretty much out of her reach and is unable to be achieved in anyway despite her gigantic size upon realizing this and what she's really been needing to do instead: Busting Phineas and Ferb, which basically resulted in neither of them keeping her (in attempt to try and get it for themselves to make money afterwards upon immediately noticing it when Candace left it with them). As before Perry fights Doofenshmirtz with the help of the fish agents to stop his usual scheme, they accidentally toss the elixir off the building and into Doofenshmirtz's hands as he puts it into the inator without realizing it and sets the effect range to be "200 yards" around his building but is accidentally set to be "universal" when he was kicked into it by Perry, causing it to make the entire universe expand unintentionally, making it look like nothing ever happened to Candace, including Baljeet's watermelon. It is unknown if the effects of it are temporary and eventually wore off or permanent and stayed until the end of time, when used on something whether is an object or a human like Baljeet's watermelon or Candace, or even the whole universe, it seems like that the process of it doesn't always take a long time for it to complete it's full effect, as when used on Baljeet's watermelon, it took a little while for it to completely take full effect and when Doofenshmirtz unintentionally used it on the whole universe with his inator without even realizing that he put the elixir in there, it all grew at once. But then again maybe the process of it may have been sped up when it was used in a machine.
When used on a human like Candace, it only seems to do just that and nothing else, otherwise it would have completely changed the way she is by not only becoming gigantic but also possibly causing her to attack anything or run amok on a destructive rampage of any kind, possibly causing a lot of destruction to Danville if it had done more than just making her grow gigantic.
Phineas later used the elixir again to make their Aqua Primates grow ("Attack of the 50 Foot Sister", "Phineas and Ferb Hawaiian Vacation"). And Candace would mention it again in the events of it from later on afterwards in songs she would sing either by being blown up to 50 feet or directly mentioning the elixir herself ("Rollercoaster: The Musical!", "Phineas and Ferb the Movie: Candace Against the Universe").