Can you help me, please. Just need examples

Modulatory neurotransmitters play different roles and functions. For example, dopamine is involved in reward/addiction, Parkinson's, and schizophrenia through different parts of the brain. This is why when you take a psychoactive drug it can have many seemingly unexpected side effects. What are some examples of how the multiple functions of a neurotransmitter could result in different effects when you take a drug to alter the levels of a particular neurotransmitter.