agent that causes hallucinations, perception anomalies, or other substantial subjective changes

Hallucinogens are chemical substances which temporarily change the way people see the world. Hallucinogens cause hallucinations. People no longer perceive reality as it is, but they see a reality that is distorted by their emotions. A person who is under the influence of a hallucionogen is said to be having a trip.

Eating from this mushroom (called fly agaric) causes hallucinations. This has been known since antiquity.
Myristica fragrans fruit, halved, showing within the red seed (nutmeg) enclosed in its brown aril (mace).

Hallucinogens have probably been used in the context of a cultic or religious function as long as there were humans.

There are different kinds of hallucinogens:

Most hallucinogens are highly addictive. This means that the user will want to take the drug over and over again. It also means that there will be some effect of getting used to the drug. In the long run, more and more of the drug will need to be taken, to get the same effect.

People that are under the unintended influence of a hallucinogen may need immediate help, as in a medical emergency.