In many superhero stories, there exists a drug that can grant or enhance a user's superpowers such as Marvel's Super Soldier Serum, and for My Hero Academia, that drug is Trigger. But where other superpower-enhancing formulas are often rare and secretive, Trigger is everywhere--even available legally.
Trigger has shown up in the My Hero Academia series many times, although details about it are usually given sparingly. It's known to be in high demand, and the sale of Trigger was a major source of funding for Overhaul and the Shie Hassaikai during that arc. The effect it has on quirks can be difficult to predict; when one street-level thug took it during a fight with Kirishima, it caused his body to erupt with blades, something that could be wildly dangerous to enemy and ally alike if the effect wasn't known beforehand. A variant being distributed in My Hero Academia Vigilantes caused people's intellectual faculties to degrade in addition to boosting their quirks, leading to what were essentially mindless monster attacks popping up anywhere at any time. Trigger can be injected or taken orally, depending on the particular iteration, and usually lasts only a brief time, although it's said the variety created in America is capable of lasting hours.
As is often the case with drugs that are abused, Trigger was developed legitimately for the purposes of boosting the power of weaker quirks, like Midoriya's mother's low-tier telekinesis. But as it turns out, it could boost a strong quirk by just as much, if not more, and those hungry for power quickly flocked to the drug. Many people in My Hero Academia seem to derive a certain level of joy from using their quirks, as seen with villainous characters like Himiko Toga, and the more powerful it makes them feel, the better it is to use. While Trigger is obviously popular among criminals, who often use it immediately when confronted by a hero, it's also used by ordinary salarymen who feel a lack of control in their lives and those on the fringes of society. Since it's been shown that quirks can be awakened and grow in power over time, one possible mechanism for its function is that Trigger prematurely accesses that power for a limited period of time. Frequent use of Trigger turns the user's tongue black, and stronger varieties may cause permanent heteromorphic/mutant changes, such as taking on animalistic traits or radically increasing in size.
My Hero Academia's creator Kohei Horikoshi is known to be a big fan of Western comics and superheroes, so Trigger does share some similarities with more famous superpower elixirs. Many of the attempts at recreating Marvel's Super Soldier Serum, such as the Ultimate versions of Hulk and Green Goblin, lead to the user losing their rationality or weakening their sanity, similar to Trigger. Its ability to amplify existing powers is also similar to the Mutant Growth Hormone seen in some X-Men comics, especially comparable since both quirks and mutant powers are (typically) possessed from birth.
One interesting difference, though, is that no heroes are ever shown to use Trigger, while similar compounds like the Super Soldier Serum are used by heroes and villains alike in most Western comics. Aizawa has been willing to look the other way when civilians use it in dire circumstances, but most heroes are very much against its use. Like many illegal substances in real life, it can be difficult to know the substance will do as described, and with the risk of turning into a Kaiju-tier rampaging monster, it's just not worth it. However, as the situation in Japan has spiraled out of control in recent chapters, it may be time for My Hero Academia's pros to consider whatever options they can to beat the villains down for good.