How do meth and heroin affect the central nervous system?

Up and down: Meth stimulates and heroin depresses. This assumes person is normal and the drugs are pure.

Opposite Effects: Methamphetamine causes activation of the central nervous system (inc heart rate and blood pressure). Heroin causes supression of the central nervous system (sedation and respiratory suppression). Both substances are highly addictive with high rates of morbidity and mortality. Due to its strong respiratory suppression, heroin is more likely to cause death. Best to avoid both entirely.