On your way to an examination you experience that vulnerable feeling that an attack of diarrhea is imminent. If you stopped at a drugstore, you could buy this antidiarrheal drug without a prescription even though it is related chemically to the strong opioid-analgesic meperidine (AIPG 2009)
a. Aluminum hydroxide is constipating but is not related chemically to meperidine; magnesium hydroxide is a strong laxative.
b. The two antidiarrheal drugs that are structurally related to opioids are diphenoxylate and loperamide.
c. Loperamide is available over-the-counter; diphenoxylate is mixed with atropine alkaloids, and the product (Lomotil, others) requires a prescription.