Poison. Non-lethal, in a way.
Now that the effect of the sedative traveled through every part of Riley's body, his tongue was also starting to recognize the taste of the poison. And right there and then, Riley realized that Nutno and everything going around him is real.
Not because he recognized the taste, no—his mind could have made that up—but because he was poisoned in the first place. Paige had told him that the hallucinations will usually have a way of revealing themselves to be just that, hallucinations, and this was it.
Unlike Paige's, his hallucinations would never even attempt to kill him because there was no way to kill him in the first place. When Diana revealed was bringing him back to Earth, and he finally remembered having Paige's abilities, he once again attempted to end his existence.
He created a poison that would kill him once and for all, and all it did was let Nothing have the chance to laugh at him again.
"..."
Bros really threw Riley to another universe and said he is their problem now.