Severe pain is often treated with opioids. Antidepressants that inhibit serotonin and norepinephrine reuptake (SNRI) have also shown a pain relieving effect, but for both SNRI and opioids, the specific mode of action in humans remains vague. This study investigated how oxycodone and venlafaxine affect spinal and supraspinal pain processing. Twenty volunteers were included in this randomized cross-over study comparing 5-day treatment with venlafaxine, oxycodone and placebo. As a proxy of the spinal pain transmission, the nociceptive withdrawal reflex (NWR) to electrical stimulation on the sole of the foot was recorded at the tibialis anterior muscle before and after 5 days of treatment. For the supraspinal activity, 61-channel electroencephalogram evoked potentials (EPs) to the electrical stimulations were simultaneously recorded. Areas under curve (AUCs) of the EMG signals were analyzed. Latencies and AUCs were computed for the major EP peaks and brain source analysis was done. The NWR was decreased in venlafaxine arm (P = 0.02), but the EP parameters did not change. Oxycodone increased the AUC of the EP response (P = 0.04). Oxycodone also shifted the cingulate activity anteriorly in the mid-cingulate-operculum network (P