Introduction: Addiction is a mental disorder that has many adverse effects on brain health. It alters brain structure and deteriorates brain functionality. Impairment of brain cognition in drug addiction is illustrated in many previous works; however, olfactory perception in addiction and, in particular, its neuronal mechanisms have rarely been studied. 
Methods: In this experiment, we recruited 20 heroin addicts and 20 normal controls of the same sex, age, handedness, and socioeconomic status and compared their brain function while perceiving non-craving odors during the functional magnetic resonance imaging (fMRI). We intended to define the default olfactory system performance in addicts compared to healthy people. 
Results: Our study showed an overall larger activation in addicts when processing olfactory stimuli. In particular, and when comparing the two groups, the right anterior cingulate and right superior frontal gyrus had higher activations than normal, whereas the left lingual gyrus and left cerebellum showed stronger activations in the addicts. 
Conclusion: The result of this study can unveil the missing components in addiction brain circuitry. This information is helpful in better understanding the neural mechanisms of addiction and may be advantageous in designing programs for addiction prevention or clinical treatment.