Final answer:
The slang term used to denote a mixture of heroin and cocaine injected into the bloodstream by the user is speedball.
Step-by-step explanation:
The slang term used to denote a mixture of heroin and cocaine injected into the bloodstream by the user is speedball. Speedball is a dangerous combination of these two drugs, which have opposite effects on the body. Heroin is a depressant that slows down the central nervous system, while cocaine is a stimulant that speeds up the nervous system. Combining these drugs can lead to serious health risks, including overdose, heart problems, and respiratory failure.