Which of the following is not considered a performance-enhancing drug?

Opiates are not typically classified as performance-enhancing drugs within the context of athletic performance. They are primarily used for their analgesic properties to relieve pain. While opiates may provide relief from pain and could potentially allow an athlete to train harder or push through discomfort, they do not directly enhance physical performance, strength, endurance, or recovery in the ways that anabolic steroids, stimulants, and blood doping do.

Anabolic steroids enhance muscle mass and strength, stimulants can increase energy, alertness, and reduce fatigue, and blood doping improves oxygen delivery to muscles, all of which contribute directly to improved sports performance. In contrast, the use of opiates does not provide the same kinds of performance benefits and is more associated with pain management rather than enhancement of athletic ability.