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Opioids are a class of compounds that elicit analgesic (pain killing) effects in humans and animals by binding to the µ-opioid receptor within the central nervous system . The following table lists opioid and non-opioid analgesic drugs and their relative potencies . Values for the potencies represent opioids taken orally unless another route of administration is provided. As such, their bioavailabilities differ, and they may be more potent when taken intravenously . Methadone is different from most opioids considering its potency can vary depending on how long it is taken. Acute use, 1–3 days, yields a potency about × stronger than that of morphine and chronic use (7 days+) yields a potency about to 5× that of morphine due to methadone being stored in fat tissue, thus giving higher serum levels with longer use. [ citation needed ] Similarly, the effect of tramadol increases after consecutive dosing due to the accumulation of its active metabolite and an increase of the oral bioavailability in chronic use; this effect becomes less significant again with even longer use as tolerance develops. [ citation needed ]