When offered one of almost any of the drugs that humans abuse (exceptions being those that distort sensations and perceptions, such as hallucinogens like LSD), the animal almost always chooses the lever that results in drug delivery. The sensation brought about by the infusion of the drug is positive and acts as a reward that positively reinforces the act of pressing the lever. Every lever press, followed by an infusion of the sensation-producing drug, further reinforces the lever-pressing behavior. The animal appears hooked, pressing the lever repeatedly.
In the case of a rewarding drug, such as cocaine, the animal might ignore food, water, or even a sexually available mate, and it presses the lever until it is too exhausted to continue. Although the animal controls the act of lever pressing, the researcher controls the total amount of drug that is administered and prevents the animal from taking enough to injure or kill itself accidentally, as sadly can happen with humans.
— The Addicted Brain (Michael Kuhar), Loc. 320–7
In the Sex and the city TV show, Amanda has a joke about the lever. The lever the men have, you know. Oh, that lever may cause so many troubles!