From the full text, they used 5 micrograms of naltrexone. Naltrexone dose is very finicky. There's low dose (LDN), very low dose (VLDN), and ultra-low dose (ULDN). This is a ULDN. I believe the type of effects I'm looking for occur at VLDN and LDN doses. If I recall correctly, I think 2 mg is optimal for analgesia, but I don't remember whether this was with or without morphine. 0.125 mg and 0.25 mg are effective in opioid withdrawal when combined with methadone. Thus, it's not too surprising that 5 mcg had little effect. At those doses, the effects it does have are like mediated by different mechanisms.