Warfarin-acetaminophen drug interaction revisited

Potential drug-drug interactions in paediatric outpatient prescriptions in Nigeria and implications for the future

BACKGROUND

Information regarding the incidence of drug-drug interactions (DDIs) and adverse drug events (ADEs) among paediatric patients in Nigeria is limited.

METHODS

Prospective clinical audit among paediatric outpatients in four general hospitals in Nigeria over a 3-month period. Details of ADEs documented in case files was extracted.

RESULTS

Among 1233 eligible patients, 208 (16.9%) received prescriptions with at least one potential DDI. Seven drug classes were implicated with antimalarial combination therapies predominating. Exposure mostly to a single potential DDI, commonly involved promethazine, artemether/lumefantrine, ciprofloxacin and artemether/lumefantrine. Exposure mostly to major and serious, and moderate and clinically significant, potential DDIs. Overall exposure similar across all age groups and across genders. A significant association was seen between severity of potential DDIs and age. Only 48 (23.1%) of these patients presented at follow-up clinics with only 15 reporting ADEs.

CONCLUSION

There was exposure to potential DDIs in this population. However, potential DDIs were associated with only a few reported ADEs.

Prothrombin time prolongation in paracetamol poisoning: a relevant marker of hepatic failure?

The association between paracetamol overdose and prolonged prothrombin time due to hepatic failure is well recognized. However, little is known of the possibility that paracetamol overdose can prolong the prothrombin time without overt hepatic failure. The few data from the literature suggest this is either due to a reduction in the functional levels of the vitamin K-dependent clotting factors by elevated doses of paracetamol, or a consequence of the administration of the antidote N-acetylcystein. The three reported cases provide further evidence that paracetamol overdose can be associated with a prolongation in the prothrombin time without overt hepatic failure. Even though the prothrombin time provides useful prognosis information, decisions regarding the management of these patients should not solely be based on this endpoint to avoid misinterpretation of the accuracy and the severity of liver failure.

Acute warfarin toxicity: An unanticipated consequence of amoxicillin/clavulanate administration

PATIENT

Male, 53 FINAL DIAGNOSIS: Acute Warfarin toxicity Symptoms: -

MEDICATION

Warfarin Clinical Procedure: - Specialty: Hematology.

OBJECTIVE

Unusual clinical course.

BACKGROUND

Warfarin remains the most common anticoagulant in the management of thromboembolic diseases. However, its extensive drug interaction requires frequent monitoring and dose adjustments. Almost all antibiotics, including penicillins, have the potential to interact with warfarin causing either under or over anticoagulation which increases the risk for thrombus formation and significant bleeding respectively.

CASE REPORT

A 53-year-old Caucasian male with a history of protein C deficiency and recurrent intravascular thrombosis developed a dental abscess. He was treated with amoxicillin/clavulanate 500/125 mg twice daily and referred to a dentist. He developed significant bleeding after tooth extraction. INR was 20.4. He received fresh frozen plasma and vitamin K with resolution of bleeding.

CONCLUSIONS

While rare, clinically significant prolonged prothrombin time and potentially life threatening bleeding can occur when amoxicillin/clavulanate is concomitantly administered with warfarin. Prompt recognition and intervention is necessary to avoid life threatening complications from warfarin-amoxicillin/clavulanate interaction.

Association of genetic polymorphisms with warfarin dose requirements in Chinese patients

OBJECTIVE

Warfarin is a commonly used anticoagulant with a narrow therapeutic range and large interindividual differences in dosing requirements. Previously, studies have identified that the interindividual variability was influenced by varieties of factors, including age, body size, vitamin K intake, interacting medications, as well as genetic variants. We aimed to investigate the effect of single-nucleotide polymorphisms (SNPs) on the interindividual variability of warfarin dose requirements in Chinese patients.

METHODS

The study population consisted of 300 patients with a stable maintenance dose of warfarin. We examined SNPs in eight genes involving in the biotransformation and mode of action of warfarin (i.e., CYP4F2, CYP2C19, APOE, CALU, EPHX1, PROC, CYP2C9, and GGCX) using the SNaPshot assay.

RESULTS

The mean daily warfarin dose in patients carrying CYP2C19 rs3814637CC, CYP2C9 rs1057910AA, and GGCX rs699664AA genotype was 3.39, 3.34, and 3.51 mg/day, respectively, which was higher than those carrying CYP2C19 rs3814637TT, CYP2C9 rs1057910CC, and rs699664GG genotype (2.00, 0.81, and 3.09 mg/day, respectively).

CONCLUSION

These findings indicate that individuals carrying the CYP2C19 rs3814637CC or CYP2C9 rs1057910AA or GGCX rs699664AA genotype needed higher warfarin doses in the Chinese population.

Effect of acetaminophen on international normalized ratio in patients receiving warfarin therapy

Warfarin is known to have extensive interactions with many classes of drugs. The literature suggesting a relevant interaction between acetaminophen and warfarin is inconsistent. Considering the ubiquitous use of acetaminophen, a review of the effects on international normalized ratio (INR) in patients taking warfarin was necessary. Thus, we performed a search of the PubMed (1966-November 2010) and International Pharmaceutical Abstracts (1970-November 2010) databases to review the available literature addressing an acetaminophen-warfarin interaction and its possible mechanisms. A sample of case reports, in addition to all English-language studies were evaluated, and relevant references were examined for additional articles. Reports of nonwarfarin coumarin anticoagulants were excluded. Published documentation reporting an interaction between acetaminophen and warfarin is limited. Small prospective studies of various designs and case studies describe aberrant INR results in patients using acetaminophen while receiving warfarin. These INR elevations typically involved acetaminophen ingestion of at least 2 g/day for several consecutive days. In several small prospective studies, INR results were elevated to a statistically significant extent that would require a change in warfarin dosing and monitoring in clinical practice. The mechanism for this interaction remains to be elucidated yet is suggested to occur through alterations in hepatic metabolism. The use of moderate-to-high doses of acetaminophen while receiving warfarin results in supra-therapeutic INRs in some patients. The characteristics that may predispose a patient to this interaction are unclear, yet the widespread use of acetaminophen calls for enhanced clinician awareness and reinforcement of patient education about this interaction.

Concomitant drug, dietary, and lifestyle issues in patients with atrial fibrillation receiving anticoagulation therapy for stroke prophylaxis

Atrial fibrillation is a common cardiac arrhythmia and the leading risk factor for stroke. In those at moderate to high risk of stroke, oral anticoagulation therapy with warfarin (a vitamin K antagonist) significantly reduces not only the frequency of such events but also their severity and the associated risk of death. However, achieving optimal anticoagulation with this agent is clinically challenging in view of its complex pharmacokinetic and pharmacodynamic profile. In this regard, concomitant drug therapy (both prescription and over-the-counter medications, including herbal products, vitamins, and various nutritional supplements), along with alcohol intake, dietary factors, and changes in lifestyle, can significantly affect anticoagulation control and thereby expose patients to the risk of bleeding or thromboembolic complications (due to over- and underanticoagulation, respectively). Therefore, it is recommended that intensified monitoring of anticoagulation be performed at initiation and discontinuation of concomitant drug therapy, and in the case of significant dietary and lifestyle changes. Moreover, many patients receive inadequate education and are unaware of such risks and their implications, highlighting the need for better awareness and education on this important aspect of anticoagulation therapy.

Does paracetamol potentiate the effects of oral anticoagulants?: a literature review

Paracetamol (acetaminophen) is the analgesic and antipyretic therapy of choice for patients receiving oral anticoagulation. It is widely used by patients in both prescription and over-the-counter products, resulting in frequent co-prescription with oral anticoagulants, especially in elderly patients. Indeed, older patients are the most likely to receive this combination of drugs because indications for both oral anticoagulation and analgesic therapy increase with age. For many years reports have presented evidence both for and against the idea that paracetamol may potentiate the anticoagulant effect of oral anticoagulants, thus increasing haemorrhagic risk in patients receiving this combination of drugs. This issue has continued to be a matter of debate in recent publications. No clear practical conclusion can be drawn from the studies because of methodological bias and the lack of clinical relevance. No prospective, randomised study assessing the effect of paracetamol on the anticoagulant effect of oral anticoagulants as used in clinical practice (i.e. the types of patients and dosages used in clinical practice) are available in the literature. The implications are considerable since on the one hand, the ingestion of paracetamol may be a cause of altered anticoagulation in patients who regularly take oral anticoagulation and who may have a haemorrhagic risk factor; and on the other hand, paracetamol might be the analgesic drug of choice that can be used without the need for any restrictions in patients receiving oral anticoagulant drugs. A comprehensive search of Medline and EMBASE for studies and case reports from 1966-2002 was performed in order to review the available literature on the interaction between paracetamol and oral anticoagulant drugs. In conclusion, the potential interaction between oral anticoagulant drugs and paracetamol is an important unanswered question, due to the growing incidence of the concomitant use of these drugs and the possible bleeding implications. The association between paracetamol and the occurrence of excessive INR values remains controversial due to lack of prospective clinical studies assessing the effect of the prescription of paracetamol in patients receiving long-term oral anticoagulation in clinical conditions. Such a study is currently ongoing.

The potential interaction between oral anticoagulants and acetaminophen in everyday practice

OBJECTIVE

The drug-drug interaction between oral anticoagulants (especially warfarin) and acetaminophen has been described, but evidence is conflicting and evidence for a similar interaction between acenocoumarol or phenprocoumon and acetaminophen is limited. Therefore, a study was performed to determine the influence of acetaminophen on oral anticoagulation with acenocoumarol or phenprocoumon in everyday practice.

METHOD

Included were patients from two Dutch anticoagulant clinics who had used oral anticoagulants for at least three months and who started with acetaminophen (case group) or benzodiazepines (control group).

RESULTS

Fifty-four patients were included who had a first prescription of acetaminophen in their pharmacy record (during oral anticoagulant use) and twenty patients with a first prescription of a benzodiazepine (during oral anticoagulant use). The INR (International Normalized Ratio) difference before and after acetaminophen use was not statistically significant between the two groups and showed no dose dependency.

CONCLUSION

These data do not demonstrate that acetaminophen, in the dosages used in everyday practice, has a clinically relevant influence on the INR in patients using acenocoumarol or phenprocoumon.

Oral surgery in patients on anticoagulant therapy

OBJECTIVE

Surgery is the main oral healthcare hazard to the patient with a bleeding tendency, which is mostly caused by the use of anticoagulants. The traditional management entails the interruption of anticoagulant therapy for dental surgery to prevent hemorrhage. However, this practice may increase the risk of a potentially life-threatening thromboembolism. Because this issue is still controversial, it is the aim of this paper to review the evidence, to highlight the areas of major concern, and to suggest management regimens for patients on the 3 main types of anticoagulants: coumarins, heparins, and aspirin. MATERIALS REVIEWED: The pertinent literature and clinical protocols of hospital dentistry departments have been extensively reviewed and discussed.

RESULTS

Several evolving clinical practices in the last years have been detected: anticoagulant use is generally not discontinued; oral surgery is performed despite laboratory values showing significant bleeding tendency; new effective local methods are used to prevent bleeding; and patients at risk are referred to hospital-based clinics.

CONCLUSION

The management of oral surgery procedures on patients treated with anticoagulants should be influenced by several factors: extent and urgency of surgery, laboratory values, treating physician's recommendation, available facilities, dentist expertise, and patient's oral, medical, and general condition.

Areas of emerging interest in analgesia: cardiovascular complications

Many people obtain symptomatic relief from acute, chronic, or recurring pain conditions by using an over-the-counter analgesic. As with the use of any drug, this involves achieving the appropriate balance between potential benefit and risk of harm. The adverse effects of aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) in the gastrointestinal (GI) tract are widely appreciated. On the basis of their pharmacology, however, these drugs also have the potential for causing adverse effects in the cardiovascular system. This is particularly the case in certain overlapping populations (eg, the elderly or those with cardiac failure, hypertension, or renal impairment). And the size of the exposed populations and the fact they comprise people likely to require pain management because of concomitant illnesses make the cardiovascular implications of analgesic use potentially a more serious issue for public health than the more widely recognized GI complications of aspirin and NSAID use. This article discusses the impact on the cardiovascular system of different classes of analgesics (NSAIDs, the new cyclooxygenase-2-selective inhibitors [CSIs], and paracetamol) in terms of cardiac function, thrombotic and cardioprotective potential, and hypertension. It identifies patients at risk for analgesic-related cardiovascular adverse events, and considers their options for managing mild-to-moderate pain. Unlike that of the NSAIDs and CSIs, the pharmacology of paracetamol provides no signal for risk of cardiovascular adverse events, and paracetamol should, therefore, be considered as first-line therapy in patients with cardiovascular disease.

Phytonadione therapy in a multiple-drug overdose involving warfarin

We cared for a patient who ingested an unknown amount of acetaminophen with zopiclone and warfarin. The only liver function test that was abnormal was an increased international normalized ratio (INR), which remained elevated despite treatment with subcutaneous phytonadione and a prolonged infusion of N-acetylcysteine. An interaction between acetaminophen and warfarin may have decreased the hepatic metabolism of warfarin. The patient received numerous antibiotics that may have contributed to the increased INR. The prolonged elevation of INR also may have been due to infrequent administration of phytonadione.