Based on observations that in rodents, acetaldehyde induces reinforcing effects acting in concert with nicotine, Talhout et al (2007) hypothesized that harmans (condensation products of acetaldehyde and biogenic amines) may be responsible for these nicotine-reinforcing effects of acetaldehyde. Harmans are formed in cigarette smoke, and blood harman levels among smokers appear to be 2-10 times higher than in non-smokers. Harmans inhibit monoamine oxidase (MAO), and like other MAO-inhibitors, these might help maintaining the behavioral sensitization to nicotine. This led these authors to speculate that acetaldehyde may increase the addictive potential of tobacco products via formation of harmans in cigarette smoke.