Harm reduction for addictive consumption: When does it improve health and when does it backfire?

Some harm reduction strategies encourage individuals to switch from a harmful addictive good to a less harmful addictive good; examples include e-cigarettes (substitutes for combustible cigarettes) and methadone and buprenorphine (substitutes for opioids). These have proven to be controversial. Advocates argue that people struggling with addiction benefit because they can switch to a less harmful substance, but opponents argue that this could encourage abstainers to begin using the harm-reduction method or even, eventually, the original addictive good. This paper builds on theories of addiction to model the introduction of a harm reduction method, and demonstrates the conditions under which each side is correct.