Anhedonia - the inability or failure to experience pleasure - is a common characteristic in drug addiction. One theory is that the drugs raise the brain's threshold for feeling pleasure so that, for example, a cup of tea and a couple of chocolate biscuits no longer have the same effect, and that this threshold does not return to normal even after people have given up taking drugs. A study of 52 people by researchers at the University of Melbourne compared 33 people being treated for heroin addiction with 19 healthy controls. Participants were shown pictures of pleasant stimuli and drug-related stimuli and their reactions to them were measured using a range of physiological methods. The heroin users demonstrated a reduced responsiveness to the pleasant stimuli and an increased response to the drug-related stimuli. How the drug users reacted to the stimuli was a significant predictor of heroin use at a six-month follow-up.
Lubman, Dan I. ... [et al] - Responsiveness to drug cues and natural rewards in opiate addiction Archives of General Psychiatry February 2009, 66(2), 205-213