Loperamide: Agonist at u-opioid receptors. Slows gut motility (remember, constipation is a common side effect for all opioids).
Q: Would a junkie want to use Loperamide?
A: No, it has poor CNS penetration (which is why it has a low addictive potential).
Q: Would a junkie rather have morphine or buprenorphine?
A: Morphine. Both are u-opioid agonists, but morphine is a full agonist while buprenorphine is only a partial agonist.
Q: What about morphine vs. codeine?
A: Trick question, both are partial agonists.
loperamide is basically diphenoxylate + atropine. atropine is added to reduce abuse potential by diphenoxylate.