Edward H. Hagen: Washington State University Vancouver Department of Anthropology

Current neurobiological theory of recreational drug use is based on the observation that all addictive drugs induce changes in activity of dopaminergic circuitry, interfering with reward processing, thus enhancing drug seeking and consumption. On this view, recreational drugs “hijack” evolved reward circuitry.

Current theory of drug origins, in contrast, views almost all major drugs of abuse, including nicotine, cocaine and opiates, as plant neurotoxins that evolved to punish and deter herbivores. According to this latter view, plants should not have evolved compounds that reward or reinforce plant consumption. Mammals, in turn, should not have evolved reinforcement mechanisms easily triggered by toxic substances. Situated in an ecological context, drug reward is a paradox.

One potential resolution of the paradox is that humans, like other animals, might have evolved to exploit plant neurotoxins to defend against pathogens. Psychoactive substances, by definition, interfere with neural function. Psychoactivity is thus a potential cue of efficacy against pathogens with nervous systems. Specifically, recreational drug use could be a form of chemoprophylaxis (preventative medicine) or chemotherapy (palliative medicine) against pathogens such as fleas, ticks, and helminths.

The chemoprophylaxis and chemotherapy hypotheses were examined in a population of Aka foragers in the Central African Republic. The Aka have high levels of helminth infection yet little access to Western anti-worm medicines, and so might be motivated (consciously or unconsciously) to consume readily available substances that are effective against worms. The plant defensive chemical nicotine, in particular, is an effective anthelmintic. Hence, “recreational” tobacco use, which is widespread at the study site, might be a form of self-medication against helminths or other pathogens.

The chemoprophylaxis hypothesis predicts that smoking should protect against helminth infection, an hypothesis tested in a cross sectional study of nicotine exposure vs. worm burden. The chemotherapy hypothesis predicts that smoking should increase upon helminth infection, and decrease when the infection abates. The latter prediction was tested in a randomized, double-blind, placebo-controlled trial involving treatment with albendazole, a commercial anthelmintic.