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Analgesic efficacy. This graph illustrates a typical dose-response curve for orally administered (PO) analgesics. The dose-response curve for opioids such as morphine demonstrates unlimited efficacy in which greater doses provide greater analgesia. At equipotent doses, all opioids demonstrate a similar dose response. In contrast, nonopioids demonstrate a “ceiling” effect that generally is adequate for relief of mild to moderate pain (pain relief rating of 4–5 in this scale). For ibuprofen, doses greater than 400 mg do not provide further analgesia. For aspirin (ASA) and acetaminophen (APAP), this ceiling effect is achieved at 1000 mg and is somewhat lower than that provided by nonsteroidal anti-inflammatory drugs (NSAIDs).

Nociceptive pathways. The drawing illustrates [A] incoming nociceptive impulses, [B] ascending nociceptive tracks, and [C] descending inhibitory tracts that act to blunt incoming pain signals. Abbreviations represent the myriad of neurotransmitters that contribute to pain transmission. They reflect potential targets for pharmacologic intervention to control pain.

Synthesis and functions of prostanoids.

Molecular structures of morphine, codeine, and derivatives. Codeine and its derivatives differ from their morphine-derived counterparts only in a methyl substitution (circled). This methyl group prevents adequate binding to the mu opioid receptor that mediates most opioid effects. However, these prodrugs are capable of producing nausea and constipation, which are problematic when high doses are administered to generate enough active metabolite for analgesia. (Asterisks indicate molecular alterations from morphine and codeine.)