“…Although the mechanism underlying the US preexposure effect (either with the same or different drugs) is not fully characterized, for some drugs, e.g., alcohol, nicotine and morphine, it is thought that changes to the drug during preexposure impact the effects of that (or a different drug) at conditioning, weakening its ability to induce aversions (Barker and Johns, 1978, Berman and Cannon, 1974, Hunt et al , 1985, Iwamoto and Williamson, 1984, Simpson and Riley, 2005). The impact of preexposure in the cross-drug design is generally thought to be the result of the development of tolerance to common aversive effects shared by the drugs, presumably due to some common mechanism of action (Riley and Simpson, 2001: Serafine and Riley 2009: Serafine and Riley 2010). It is interesting to note that the cross-drug preexposure effect has been previously reported with nicotine and alcohol (Kunin et al , 1999).…”