The Pym Particle is one of the many catch-all plot devices in the Marvel Cinematic Universe. Like Vibranium or the six Infinity Stones, their potential is undefined, allowing screenwriter's a lot of wiggle room to create new quantum technologies as needed. However, in the first Ant-Man film, Hank Pym outlines a basic limit of the Pym particle that could lead to dire ramifications for many heroes.
Pym explains that the particle is volatile and that exposure to it without his specialized helmet can alter the brain's chemistry. When Scott Lang asks why Pym can no longer use the Ant-Man suit himself, he says that after many years the suit took a toll on him. So even with the helmet, the particle can have lasting effects on its users. In that case, the Pym particle may be slowly killing its creator, and potentially the entire Avengers team, who used the particle to travel through time in Avengers: Endgame.
The situation for our heroes could be even worse than long-term sickness. Pym's former assistant and the first Ant-Man film's main antagonist, Darren Cross, spent years attempting to crack his formula. Prolonged exposure during his experiments led Cross to become unstable and violent due to his altered brain chemistry. While we know Pym was mostly protected thanks to his suit, there's the potential that he could lose control much like Cross.
In Ant-Man and the Wasp, the screenwriter's played a little fast and loose with the Pym particle. Characters like Pym, Lang, Hope van Dyne, and Luis, were all shrunk inside a car without wearing any protective gear on their person. While it's not fair to say that the writers forgot the rule that had been established in the first film, so far we've yet to see any real repercussions like we did with Cross. However, the Marvel Cinematic Universe executes much of its storytelling on long term payoff just like this. So it's reasonable to speculate that we could see some consequences to the Pym particle in the recently announced Ant-Man 3.
We know Pym has been weakened by the particle, and he's already been shown to be a little abrasive and rude, even to those close to him. While it's unlikely we'll see Pym go down the dark and abusive path he has in some of the comics, it is still fair to say that there is some precedent for a more villainous version of the character. After all, we've yet to see how he is handling both his trip to the quantum realm in Ant-Man and the Wasp and his resurrection after the blip in Avengers: Endgame. With an unstable and altered brain and more than enough trauma to lead to possible PTSD, Pym could be a very different man when he appears in Ant-Man 3.
While Pym may be the first and most obvious candidate to be driven insane by the Pym particle, he's not the only one. As previously mentioned, the entire Avengers roster who survived the blip used the particles to time travel in Avengers: Endgame. While they weren't exposed to the particle as long as Pym, or even Lang, it's unclear if using the particle in that manner could have more serious effects. After all, we saw in Ant-Man and the Wasp that the quantum realm can have some strange effects on those who travel through it. Janet van Dyne returned from the quantum realm with brand new abilities that allowed her to heal the villain Ghost by harnessing quantum energy.
Speaking of van Dyne, she may be the next in line to feel the effects of altered brain chemistry. After reuniting with Pym in Ant-Man and the Wasp she says, "I'm not the same woman I was 30 years ago, Henry. This place... it changes you." So far we've only seen her years in the quantum realm manifest in new superpowers, but it's possible there could be direr side effects just waiting to be revealed. Additionally, prior to her time in the quantum realm, van Dyne had years of Pym particle exposure. She was going on missions as the Wasp for almost as long as Pym was the Ant-Man.
At this point, we really don't really know how intense the effects of Pym particles are. While Cross may have become a deeply cruel villain in Ant-Man due to exposure, it's worth keeping in mind that he was dealing with unrefined variations on Pym's formula. Also, as Pym says to Lang, "he's not the most stable guy to begin with." So, while Pym may be at great risk of going insane, and van Dyne as well, it's not easy to say whether Lang, Hope van Dyne, or the rest of the Avengers could be going down that path as well. Nonetheless, Marvel may have opened up a massive can of worms by exposing the entire team to Pym particles in Avengers: Endgame, and we'll have to wait a few more years to see the results manifest.
Ant-Man 3 will reportedly premiere in 2022 and be directed by Peyton Reed. Paul Rudd, Michael Douglas, and Evangeline Lilly are expected to reprise their roles from the previous films.