Tramadol possesses weak agonist actions at the μ-opioid receptor, releases serotonin, and inhibits the reuptake of norepinephrine.

While its action is not like that of other opioids, tramadol is a synthetic analog of the phenanthrene alkaloid codeine. Tramadol is converted to O-desmethyltramadol, a significantly more potent μ-opioid agonist. Opioids are chemical compounds which act upon one or more of the human opiate receptors. The euphoria and respiratory depression of opioids are mainly caused by the μ₁ and μ₂ receptors; the addictive nature of tramadol, as well as other opioids, is due to these effects, but tramadol's serotonergic and noradrenergic effects may contribute to possible dependence as well.^{[citation needed]} The opioid agonistic effect of tramadol and its major metabolite(s) are almost exclusively mediated by the substance's action at the μ-opioid receptor. This characteristic distinguishes tramadol from many other substances (including morphine) of the opioid drug class, which generally do not possess tramadol's degree of subtype selectivity.

Tramadol is used similarly to codeine, to treat moderate to moderately severe pain.^[5] Pharmacologically, Tramadol is similar to levorphanol (albeit with much lower μ-agonism), as both agents are also NMDA-antagonists with SNRI activity. Dextropropoxyphene (Darvon) & M1-like molecule tapentadol (Nucynta, a new synthetic atypical opioid made to mimic the agonistic properties of tramadol's metabolite, M1(O-Desmethyltramadol)) also have similar activities. Tramadol is also molecularly similar to venlafaxine (Effexor) and has similar SNRI effects, with antinociceptive effects. It has been suggested that tramadol could be effective for alleviating symptoms of depression, anxiety, and phobias^[6] because of its action on the noradrenergic and serotonergic systems, such as its "atypical" opioid activity.^[7] However, health professionals have not endorsed its use for these disorders,^[8][9] claiming it may be used as a unique treatment (only when other treatments failed), and must be used under the control of a psychiatrist.^[10][11]

In May 2009, the United States Food and Drug Administration issued a Warning Letter to Johnson & Johnson, alleging that a promotional website commissioned by the manufacturer had "overstated the efficacy" of the drug, and "minimized the serious risks".^[12] The company which produced it, the German pharmaceutical company Grünenthal GmbH, were alleged to be guilty of "minimizing" the habituating nature of the drug, although it showed little abuse liability in preliminary tests.^{[citation needed]} The 2010 Physicians Desk Reference contains several warnings from the manufacturer, which were not present in prior years. The warnings include stronger language regarding the habituating potential of tramadol, the possibility of difficulty breathing while on the medication, a new list of more serious side effects, and a notice that tramadol is not to be used in place of opiate medications for addicts. Tramadol is also not to be used in efforts to wean addict patients from opiate drugs, nor to be used to manage long-term opiate addiction.