Heroin is sometimes smoked with crack cocaine. This is an even more intensely rewarding experience than taking either drug alone. Yet in a clinical setting and among the terminally ill, the simultaneous use of cocaine, methylphenidate or amphetamines with heroin or morphine can augment the opioid's analgesic and anxiolytic effect while allowing its dosage to be lowered. The risks of respiratory depression are thus diminished. Heroin mimics the action of natural chemicals, endorphins, produced by the body in response to pain.
Endorphins are small-chain peptides that activate our endogenous opiate receptors.
The highest density of opiate receptors is found in the limbic system. Their activation produces feelings of happiness, relaxation, fearlessness and tolerance to pain. Endorphins are hundreds or even thousands of times more potent than morphine on a molar basis. Their potency means their concentrations in vivo are low. Endorphins are also involved in respiration, nausea, vomiting, pain modulation, hormonal regulation and itching.