Opioids participate in a broad spectrum of regulatory effects. The discovery of the opioid receptor system led to the initial belief that all of the observed effects in this system were associated with receptor activation. However, it must be considered that certain opioid properties are the result of the properties of other chemicals and their distribution. The presence of a tyramine moiety in opioids is suggestive of their potential antioxidant properties. Therefore, this study evaluated the antioxidant properties of opioids that are not related to opioid receptor activation. The morphine antioxidant capacity (IC 50 = 81 μM) was 2.8 times lower than that of the reference ascorbic acid (IC 50 = 29 μM). Surprisingly, the biphalin antioxidant capacity (IC 50 = 8 μM) was 3.6 times higher than that of ascorbic acid and over 10 times higher than that of morphine. This unexpectedly high biphalin antioxidant capacity correlates with its neuroprotective properties.