Cocaine acts as a stimulant, whereas heroin/morphine acts as a depressant. Co-administration is meant to provide an intense rush of euphoria with a high that is supposed to combine the effects of both drugs, while hoping to reduce the negative effects, such as anxiety, hypertension, palpitations and other common side effects of stimulants and sedation/drowsiness from the depressant. While this is somewhat effective, as one drug (the CNS stimulant) triggers the sympathetic nervous system and the other (the CNS depressant) triggers the parasympathetic nervous system, the two systems that regulate the fight-or-flight and rest-and-digest responses, respectively, and simultaneous activity of the two pathways is what normally keeps one's body in natural homeostasis, there is an imperfect overlap in the effects of stimulants and depressants. Additionally, by suppressing the typical negative side-effects of the two drugs, the user may falsely believe they have a higher tolerance, or that they are less intoxicated than they actually are. This can cause users to misjudge the intake of one or both of the drugs, sometimes fatally.
Because the stimulant effects of cocaine wear off far more quickly than the depressant effects of heroin or morphine, fatal respiratory depression often occurs when the full effects of a heroin or morphine overdosage are felt in isolation. Due to the countering effect of the cocaine, a fatally high opioid dose can be unwittingly administered without immediate incapacitation, thus providing a false sense of tolerance until it is too late. This form of delayed opioid overdose is believed to be the most common mechanism of death in speedball overdoses.
^Spears, Richard (1981). Slang and Euphemism : A Dictionary of Oaths, Curses, Insults, Sexual Slang and Metaphor, Racial Slurs, Drug Talk, Homosexual Lingo, and Related Matters. Middle Village, New York: Jonathan David. ISBN978-0-82460-259-8.