Merrybean Psychosis

The plant gets its name from the effect it has on a person's mood. Shortly after consumption, the user starts to evaluate observations more positively. Bad experiences are likely to be ignored while good ones are emphasized. In small doses, this makes it useful for pain medication.
  However, people who consume higher amounts will see drastic shifts in their emotional balance and judgment. They often grow addicted to the blissfully unaware state while trying to avoid the unpleasant aftermath. This, in turn, leads them to consume higher doses which slowly alters the way their mind works.

Stage 1

After the intended effect wears off, the emotions that the user tried to avoid come back in full force. The contrast can make the experience extremely painful, especially if they did anything under the influence that they regret. The pendulum then often swings in the other direction, emphasizing the negative while ignoring positive observations.

Stage 2

Repeated Merrybean abuse will alter the reasoning abilities in everyday life, even after the user has sobered up and recovered from the immediate effects. The optimistic part of their mind gradually splits from the pessimistic part while the ability to reason in a neutral, fact-based way is slowly eroded. Consequently, a Merrybean addict becomes prone to extreme mood swings.

Stage 3

After a few years, the divide between the optimistic and pessimistic reasoning has deepened so much that the addict stops perceiving them as part of their own mind. Victims of that drug often report hallucinations, such as voices that try to convince them of conflicting interpretations.

Stage 1 symptoms will typically fade away on their own, and the user's mood will return to an appropriate state after a while.
  Stage 2 symptoms, however, typically occur after an addiction was formed. At this point, the affected person needs to be weaned off the drug before the psychosis can be treated. This has to be done with utmost care because the mood swings will get worse by the withdrawal and can pose a threat to both the addict and the people around them. Afterwards, systematic training will be needed to rebuild their reasoning abilities.
  Once a person reaches stage 3, the treatment becomes extremely risky. The dosage has to be reduced in very small steps, and the addict needs to be monitored at all times. Their behavior might be downright unpredictable, and the negative thoughts are likely to take over as the positive thoughts have come to depend on the drug.