A reduced effectiveness of the respiratory system which can result in pneumonia, and other respiratory problems including respiratory arrest can be caused by:

A. hallucinogens: These primarily affect perception and cognition, not respiratory depression. B. PCP: PCP (phencyclidine) is a dissociative anesthetic that can cause agitation, hallucinations, or psychosis, but it doesn't typically cause respiratory depression. C. opiates: Opiates (e.g., heroin, morphine) depress the central nervous system, particularly the brain's respiratory center, which can lead to respiratory failure and death. D. cocaine: Cocaine is a stimulant that can cause cardiac issues or respiratory distress via vasoconstriction, but it doesn't suppress the respiratory drive like opiates do.