Stimulants and hallucinogens have been used to induce euphoria and alter consciousness for centuries. Cocaine and marijuana are two of the most commonly used drugs today. Polydrug use involving concurrent use of several drugs with different pharmacologic effects is increasingly found. Sometimes one drug is used to enhance the effects of another, as with the combined use of cocaine and nicotine, or cocaine and heroin in methadone-maintained pts. Some forms of polydrug use, such as the combined use of IV heroin and cocaine, are especially dangerous and account for many hospital ER visits. Chronic drug use is associated with immune system dysfunction and increased vulnerability to infections, including risk for HIV infection. The concurrent use of cocaine and opiates (“speedball”) is frequently associated with needle sharing by people using drugs intravenously.