Cross-dependence in the context of addiction refers to the phenomenon where one substance can substitute for or replace another substance in maintaining physical dependence. It occurs when the use of one substance leads to the development of tolerance, such that a greater amount of the substance is needed to achieve the same effect. If a person then switches to a different substance, they may find that they can achieve the same effect with a lower dose of the new substance, indicating that the two substances have cross-tolerance. This can occur with drugs that act on the same receptors in the brain, such as opioids and benzodiazepines, or with drugs that have similar effects, such as alcohol and other depressants. Cross-dependence can make it more difficult for an individual to quit using drugs, as switching from one substance to another may not necessarily lead to abstinence.