rats given unlimited access to cocaine for six hours per day gradually increase their lever-pressing over time to the point of physical exhaustion and even death. Increased self-administration under extended access conditions (six hours) has also been observed with methamphetamine, nicotine, heroin, and alcohol. However, rats who have access to cocaine for only one hour per day use steady amounts of cocaine over many consecutive days. That is, they don’t press the lever for more drug per unit time with each consecutive day.
