Animal studies suggest that drug effects can act as conditioned stimul
i for various unconditioned stimuli including the effects of other dru
gs. The current study investigated drug-drug conditioning in human sub
jects. Sixteen subjects were given subcutaneous injections of either n
icotine or saline before consumption of an alcoholic or soft drink in
each of eight sessions. Across sessions the content of the injections
was established as a reliable predictor of the alcoholic content of th
e drink. Physiological, subjective, and behavioural responses to the i
njections were used as indices of conditioning. Skin conductance measu
res obtained following the injections changed across trials in a way c
onsistent with a conditioned response though patterns of change on car
diac inter-beat interval were less clear. However, neither behavioural
nor subjective measures showed conditioning effects. In view of the n
umber of variables studied the evidence for the development of conditi
oned responding on physiological measures must be suspected of being a
type I error and is in need of replication. Subjects' reports reveale
d that nicotine and saline injections were difficult to discriminate.
This would have weakened conditioning effects. Suggestions are made fo
r improvements in the design of future studies of drug-drug conditioni
ng in human subjects.