A comprehensive literature search in the Medline electronic database for articles published between January 1980 and January 2016 was conducted for tramadol and the following keywords: pain and meta-analyses or Cochrane review, formulations, epidemiology, pharmacology, metabolism, cytochrome P450 (CYP), pharmacokinetics, drug interaction, clinical decision supports, pharmacodynamics, adverse effects, seizures, ondansetron, withdrawal, legal regulation, illicit use, abuse potential, and evaluation and treatment. Literature reviewed includes original research studies, retrospective case studies, toxicology data, chemical analysis studies, and federal and international regulation documents. Case studies involving individual use were examined and compiled for sections on adverse events. News articles identifying trends in international tramadol use were also included. Two of the authors evaluated the titles and abstracts of all collected publications and then further assessed the full text to be considered for inclusion in the review.
Tramadol Prescription Trends
Although the total number of opioid prescriptions in the United States has increased, the IMS Health National Prescription Audit revealed that between 2007 and 2011, the percent increase for tramadol (65%) was significantly greater than that of oxycodone/acetaminophen (24%) and hydrocodone/acetaminophen (13%) preparations. In 2013, tramadol ranked second in the total US opioid market sales at 14.7%, between hydrocodone/acetaminophen (46%) and oxycodone/acetaminophen (13.6%). Factors contributing to this increase include the prescribers' impression that tramadol has low addiction liability and a favorable safety profile. Indeed, the reports of tramadol overdose are limited, but there are multiple case reports of adverse events because of its unique pharmacology, as discussed below.
Tramadol was the first medication in its class to produce dual-analgesic effects, acting synergistically as an opioid agonist and monoaminergically as a serotonin and norepinephrine reuptake inhibitor. It acts on the μ-opioid receptor as a weak agonist and acts on serotonergic and noradrenergic nociception. Tramadol has 2 chiral centers and is used as a 1:1 racemic mixture of 2 enantiomeric diastereomers, the R,R-enantiomer ([+]-tramadol) and S,S-enantiomer ([-]-tramadol). The (+)-tramadol enantiomer is the most potent serotonin reuptake inhibitor, whereas the (-)-tramadol enantiomer is the most potent norepinephrine and serotonin reuptake inhibitor. By independently enhancing noradrenergic and serotonergic activity, they work together to produce effects of analgesia in the central nervous system (CNS).
Tramadol is converted by CYP450 enzymes 3A4 and 2D6 into 3 major metabolites, 2 of which are active. (+)-Tramadol and (+)-M1 metabolites both bind to the μ-opioid receptor to produce most of its opioid analgesic effects.[15–17] However, (+)-M1 is a high-affinity ligand and produces more potent analgesic effects than the parent compound, which is a low-affinity opioid agonist. There are some enantiomeric differences in analgesic potency of the 2 enantiomers of M1, with the (+)-O-desmethyltramadol configuration about 100 times greater than that of the (-) configuration.[15,18,19] The second metabolite, N,O-desmethyltramadol metabolite (M5), is also active and contributes to the analgesic effects. Because of the high affinity of (+)-M1 for the μ-opioid receptor, its concentration, in combination with that of the M5 metabolite, is primarily responsible for the analgesic effects of tramadol.
As CYP450-mediated phase I metabolic reactions are slower than phase II conjugation reactions, they become rate limiting in the overall metabolic disposition of CYP substrate drugs. The phase I metabolism of tramadol, shown in Figure 1, is catalyzed by CYP2D6 and CYP3A4, with the O-demethylation reaction to the active M1 metabolite catalyzed by CYP2D6. Approximately 80% of tramadol is metabolized by CYP2D6, an easily saturated, low-capacity, high-affinity enzyme that represents only 1% to 5% of the liver CYP content. Because the metabolizing capacity of patients with hepatic impairment may be significantly reduced, toxicity at the recommended dose may occur, but this has not yet been studied in patients with liver disease. Although metabolized in the liver, unchanged tramadol and its metabolites are mainly excreted in urine. In renal impairment, there are reports of decreased clearance and a 2-fold increase in the half-life of tramadol and the M1 metabolite.[21,22] Because only 7% of an administered dose is removed by dialysis, patients may receive their regular dose of tramadol.
The N-demethylation to the inactive metabolite, N-desmethyltramadol (M2), is catalyzed by CYP2B6 and CYP3A4. CYP3A4 is responsible for the metabolism of 50% of all drugs; although it exhibits polymorphisms and is subject to induction and inhibition by other substrates, few significant drug interactions between tramadol and CYP3A4 substrates have been reported.[23–25]
CYP Genetic Polymorphisms
Tramadol is bioactivated to M1, the main opioid metabolite, by CYP2D6, and there is a significant variability in the efficiency and amount of CYP2D6 enzymes among individuals. The large phenotypic variation affects the speed of metabolism and the rate of accumulation or elimination. There is increasing widespread clinical use in grouping people based on their CYP2D6 profile as follows: very low (PM or poor metabolizers) with little or no CYP2D6 function; intermediate metabolizers (IM) between poor and extensive enzymatic activity; extensive metabolizers (EM), defined as the most common level of activity; and very high (UM or ultra-metabolizers) that express multiple functional copies of the CYP2D6 gene. The effect of the CYP2D6 group activity on the opioid analgesic potency and side effect profile of tramadol was demonstrated in a pharmacokinetics study by Kirchheiner et al. The CYP2D6 gene duplication was evaluated following an 100-mg oral dose of tramadol. The bioavailability of the active M1 metabolite was found to be about 3% of the administered dose in PMs. Notably in the PM group, assays identified the highest concentrations of the parent tramadol compound. In contrast, bioavailability of the M1 metabolite was 63% in EMs and 86% in UMs. Consistent with these pharmacokinetic differences, UMs exhibited a stronger opioid response including increased pain tolerance, greater miosis, and a higher frequency of nausea compared with EMs. Pharmacogenetic testing for tramadol has historically been used to explain inefficacy or toxicity. The goal of personalized medicine is advancing with the increase in availability of commercial pharmacogenetics testing kits. Drug-metabolizing enzymes represent a major target for research and testing, and panels are available to analyze the metabolism of psychotrophic and opioid medications.
Testing has identified notable racial, ethnic, and regional patterns in the prevalence of CYP genetic polymorphisms. The PM phenotype is more frequently found in African Americans, followed by 6% to 10% in the Caucasian populations with the smallest percentage found in Asian populations (1% to 2%). Conversely, UMs are concentrated in Egypt, Iran, Saudi Arabia, and Northeast Africa where tramadol-related opioid analgesic effects, addiction, nausea, and respiratory depression are more frequently reported.[26,28–30]
Tramadol is available in the United States only as an oral preparation. Outside the United States, it is also available in a suppository and parenteral form. Formulations include immediate-release (IR) tablets (50 mg), sustained-release (SR) tablets (100, 200, and 300 mg), and extended-release (ER) capsules (100, 200, and 300 mg). A tramadol hydrochloride/acetaminophen tablet preparation (37.5 mg tramadol HCl and 325 mg acetaminophen) is also available. The recommended maximum dose of IR is 400 mg/day and the maximum dose of the ER form is 300 mg/day. Brand names in the United States include Ultram, Ultram ER, Ultracet, ConZip, Ryzolt, and Rybix orally dissolving tablet (ODT).
For the IR preparations, the effects peak at about 3 hours after administration, but can persist for 5 to 7 hours. Mean bioavailability of IR tramadol is about 70% with a half-life of about 6 hours. The SR and ER formulations increase the bioavailability and offer more stable plasma concentrations. The tablets provide a peak concentration at about 12 hours with an elimination half-life of about 9 hours. The ConZip® ER capsules contain an IR tablet with multiple SR pellets. This preparation has biphasic release: 25% within the first 2 hours, and 75% gradually released over 24 hours with a peak plasma level at 9 hours.
CYP2D6 enzymes contribute to the metabolism of approximately 25% of all medications, many of which are commonly administered to hospitalized patients receiving tramadol such as antiarrhythmics, antiemetics, antidepressants, antipsychotics, analgesics, and tamoxifen. The nature of certain tramadol drug–drug interactions involve overlapping pharmacodynamic, pharmacokinetic, and pharmacogenetic risk factors that require an appreciation of the possible interactions with certain classes of medications.
Tramadol–Ondansetron Interactions. Nausea is a prominent adverse event associated with the initial oral or IV tramadol treatment; therefore, it is important to elucidate the potential interaction between tramadol and antiemetics. In addition, most antiemetic medications that are used to prevent postoperative- and chemotherapy-induced nausea and vomiting are 5-HT3 receptor antagonists, whereas tramadol decreases serotonin reuptake. These opposing serotonergic effects increase the risk of a pharmacodynamic interaction. In addition, the "setrons," including ondansetron, dolasetron, and palonosetron (with the exception of granisetron), are partially metabolized by CYP2D6, increasing the concern for a pharmacokinetic interaction. A review of the literature suggests that the concurrent use results in a common reduced response: tramadol is a less potent analgesic, and ondansetron is less effective as an antiemetic.[18,33–40] A recent systematic review by Stevens et al supports the presence of a drug interaction with ondansetron in the early postoperative period that potentially decreases the effectiveness of tramadol; however, this likely to be less problematic in the United States where IV tramadol is not used postoperatively.
Coadministration of tramadol with proserotonergic medications can result in a hyperserotonergic state that develops soon after initiation or dosage changes of the offending agent. Serotonin syndrome (SS) can be subacute or chronic and range from mild to severe. In mild cases, patients are afebrile and may report symptoms of diarrhea, tremor, tachycardia, and autonomic findings such as shivering, diaphoresis, or mydriasis.
In severe cases, neuromuscular hyperactivity, autonomic hyperactivity, altered mental state, gastrointestinal symptoms, and even death have been reported. Serotonergic medications that can interact with tramadol include SSRIs, SNRIs, tricyclic antidepressants (TCAs), and triptans (eg, sumatriptan), antipsychotics, anticonvulsants, antiparkinsonian agents, cough and cold medications containing dextromethorphan, herbal products containing St. John's wort, and medications that inhibit the metabolism of serotonin, such as monoamine oxidase inhibitors (MAOIs).
Although we were unable to find anesthesia literature discussing SS in tramadol-treated patients, there are reports from medical, psychiatric, and emergency medicine journals. In summary, the risk for SS increases with higher dosages of both tramadol and the proserotonergic medication. Inhibition of CYP2D6 enzymes by SSRIs prevents the hepatic metabolism of tramadol. This elevates the concentration of the parent compound and increases its serotonergic effects in the brain (Table 2). SSRIs that are strong inhibitors of CYP2D6, such as sertraline,[44,45] paroxetine,[46,47] and fluoxetine, increase the risk of serotonin syndrome when taken with tramadol. CYP2D6 plays a secondary role in the metabolism of citalopram, and 2 case reports describe tramadol–citalopram-associated serotonin syndrome.[48,49] A review of the literature by Nelson and Philbrick confirms that the risk is enhanced in CYP2D6 PMs, as would be expected because they have the greatest concentration of the parent compound. In terms of treatment of serotonin syndrome, all serotonergic agents should be discontinued. Supportive care aimed at normalizing vital signs should be administered, such as oxygen given to normalize oxygen levels and IV fluids to hydrate and treat hyperthermia. If temperatures are higher than 41°, the patient should be intubated with induced neuromuscular paralysis. Muscle relaxants such as benzodiazepines (valium, lorazepam, or diazepam) may be administered to control seizures, agitation, and muscle stiffness. In extreme cases, serotonin-production blocking agents like cyproheptadine may be given. Excellent reviews of the treatment of serotonin syndrome are provided by Sporer and Frank.[48,49]
Many clinicians are unaware that tramadol can increase a patient's risk for seizure via lowering the seizure threshold. A comparison of tramadol and tapentadol exposures reported to the Data System of the American Association of Poison Control Centers between 2009 and 2014 revealed that individuals exposed to tramadol (8566 cases) identified significantly higher rates of seizures and vomiting, whereas tapentadol was associated with more classical opioid agonist reports such as respiratory depression.[51,52] Generalized seizures may occur within the first 24 hours after administration. Notably, seizures occurred both at therapeutic and supratherapeutic ranges in individuals with and without the history of a seizure disorder. Risk factors for seizures compiled from case reports include the history of traumatic brain injury and seizure activity secondary to hypoxia; administration with other medications that lower seizure threshold such as antipsychotic medications; high-dose ingestion; and coingestion with substances likely to cause drug–drug interactions.[54–56] To decrease the risk of seizure occurrence, an alternative analgesic for pain management should be selected for seizure-prone patients, and those being administered combinations of multiple medications that lower the seizure threshold should be carefully monitored. In addition, those undergoing simultaneous withdrawal from tramadol and other substances should be monitored, with gradual decreases in tramadol dosage to reduce the likelihood of a withdrawal seizure.
Tramadol has been reported to have distinctive opioid and SNRI-associated withdrawal symptoms that can occur as a result of abrupt cessation of IR and ER formulations in both abusers and patients receiving therapeutic doses for pain management. Common opioid and atypical SNRI-like withdrawal symptoms are presented in Table 1. Clinically these atypical discontinuation features have been considered similar to those of the SNRI venlafaxine and have been reported in sensitive individuals irrespective of the dosage, but generally remit with gradual dose reduction. High doses of tramadol may be tapered in the preoperative period if time permits. Although there is no literature supporting a particular tapering schedule, it is important to monitor for both typical opioid and atypical withdrawal symptoms. A case report suggested the use of lorazepam and clonidine for symptomatic relief of withdrawal symptoms.
Illicit use of Tramadol
Despite previous assumptions that tramadol did not have an addiction liability, various English- and Arabic-language studies concluded that tramadol produces desirable euphoric, stimulant, and relaxing effects that increase its abuse potential.[58–63] The most frequent abusers of tramadol are those with easy access and history of substance abuse, patients with chronic pain, and health professionals. Although rarely a primary drug of choice, a review of physician health program records showed that tramadol was the third most frequently mentioned opioid and it exceeded the abuse liability of fentanyl, oxycodone, and hydromorphone in this group of physicians. Increased monitoring is necessary in patients with a history of tramadol abuse or dependence because they are more likely to exhibit toxidromes and continue use to avoid opioid and SNRI withdrawal.
Anesth Analg. 2017;124(1):44-51. © 2017 International Anesthesia Research Society