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PubMed2004

Paracetamol (acetaminophen) warfarin interaction: NAPQI, the toxic metabolite of paracetamol, is an inhibitor of enzymes in the vitamin K cycle.

Paracetamol (acetaminophen) is generally considered to be the analgesic of choice for patients undergoing oral anticoagulant therapy. Occasionally, however, interactions have been reported with therapeutic doses of the analgesic, e.g. if the drug is taken for a longer period of time. The mechanism of this interaction is not clearly understood. We investigated the effects of paracetamol and its toxic metabolite N-acetyl-para-benzoquinoneimine (NAPQI) on in vitro vitamin K-dependent gamma-carboxylase (VKD-carb) and vitamin K epoxide reductase (VKOR) activities. Paracetamol had no effect in either enzymatic reactions. NAPQI, on the other hand, appeared to interfere with VKD carb activity via two mechanisms; 1) oxidation of the cofactor vitamin K-hydroquinone, 2) inactivation of the enzyme. The inactivation, in micromolar ranges, is not reversible and may be the result of covalent binding of NAPQI with functional amino acids. NAPQI also inhibited VKOR, but at higher concentrations. Unexpectedly, N-acetylcysteine was found to inhibit VKOR activity at concentrations that are obtained during rescue therapy of paracetamol intoxication. We conclude that, the potentiation of the oral anticoagulant effect by paracetamol is likely to result from NAPQI-induced inhibition of enzymes of the vitamin K cycle, particularly VKD-carb.

Acetaminophen↗

PubMed1985

The relative bioavailability of paracetamol after rectal administration of suppositories containing a mixture of paracetamol, codeine phosphate and buclizine hydrochloride in healthy volunteers.

'New' and 'old' suppositories (6 months and 30 months since manufacture) containing 800 mg paracetamol, 16 mg codeine phosphate and 12.5 mg buclizine hydrochloride in an identical base were administered to 10 normal volunteers at an interval of 2 weeks. Blood samples were taken at intervals up to 300 minutes after administration for estimation of paracetamol plasma concentrations using high pressure liquid chromatography. Mean peak concentrations were obtained of 4.75 +/- 0.74 mg/ml at 1.75 hours with the new suppositories and of 4.6 +/- 0.67 mg/ml at 2.0 hours with the old suppositories. The difference was not significant. Mean elimination half-life was 4.4 +/- 0.42 hours and 3.73 +/- 0.28 hours, respectively. Again, the difference was not significant, indicating that the absorption characteristics for the suppositories did not appear to deteriorate with ageing for 24 months. Bioavailability data for paracetamol derived from the results were similar to those reported by other workers who studied suppositories containing paracetamol as the only active ingredient. This indicates that the inclusion of codeine phosphate and buclizine hydrochloride in the suppository formulation investigated in the present study did not affect adversely the absorption of paracetamol.

Acetaminophen↗

PubMed1977

A double-blind comparative evaluation of aspirin, paracetamol and paracetamol + caffeine (finimal) for their analgesic effectiveness.

A double-blind, cross-over trial was made of three analgesic preparations--paracetamol, paracetamol with caffeine (Finimal) and aspirin in the relief of postoperative pain in 72 orthopedic inpatients and in 144 ambulatory outpatients suffering form common idiopathic headache. The combination of paracetamol and caffeine (Finimal) in this study shows the greatest pain relief in both groups of patients. This evaluation supports the results of BOOY3 demonstrating the superiority of the paracetamol-caffeine combination to paracetamol alone or aspirin.

Acetaminophen↗

PubMed2006

A pharmacokinetic study investigating the rate of absorption of a 500 mg dose of a rapidly absorbed paracetamol tablet and a standard paracetamol tablet.

OBJECTIVE: A rapidly absorbed tablet formulation of paracetamol containing sodium bicarbonate (PS) has been previously shown to be absorbed at least twice as fast as a standard paracetamol tablet (P) at a 1 g dose. In South America and Asia it is customary for patients to take a 500 mg dose of analgesic. The objective of this pharmacokinetic study was to compare the rate of absorption of PS versus P at a 500 mg dose. RESEARCH DESIGN AND METHODS: An open, randomized, single dose, cross-over study. Thirty Hispanic healthy volunteers randomly received a 500 mg dose taken orally with 50 mL of water 2 h after a standard breakfast. Blood samples were taken up to 10 h post-dose. Plasma concentrations of paracetamol were determined by HPLC with UV detection. MAIN OUTCOME MEASURES: AUC(0-30 min), C(plasma 30 min) and T(max) were analyzed non-parametrically by the Wilcoxon's rank sum test. A linear mixed effects model was used to analyze the logarithmically transformed AUC(0-alpha) and C(max). Bioequivalence was accepted if the 90% confidence intervals (CI) for the ratio of the means of the primary pharmacokinetic variable AUC(0-alpha) lay completely within the range 0.80-1.25. RESULTS: AUC(0-30 min) and C(plasma 30 min) were significantly greater and T(max) was significantly shorter (all p < 0.0001) for PS versus P. The formulations were bioequivalent for AUC(0-alpha) (90% CI 0.99:1.05) and no statistical difference was seen for C(max) (95% CI 0.91:1.14). CONCLUSIONS: Paracetamol was absorbed at least twice as fast from PS compared to P at a 500 mg dose. The extent of absorption was equivalent for both formulations.

Absorption↗

PubMed2003

Plasma paracetamol concentrations after different doses of rectal paracetamol in older children A comparison of 1 g vs. 40 mg x kg(-1).

We compared the plasma paracetamol levels attained after the administration of different doses of rectal paracetamol in healthy children to see if they attained therapeutic levels (> 10 microg x ml(-1)). We recruited 24 children (ASA I-III) over 25 kg undergoing elective surgery. They were randomly assigned to receive rectal paracetamol at a dose of 1 g or 40 mg x kg(-1). Blood samples were taken for analysis at 2, 3, 4 and 5 h post dose. Most children in the 1 g group failed to attain therapeutic plasma levels, whereas those in the 40 mg kg(-1) group did achieve therapeutic levels (mean maximum concentration = 7.8 vs. 15.9 microg x ml(-1), p = 0.009).

Acetaminophen↗

PubMed1997

Comparison of the efficacy of paracetamol versus paracetamol, codeine and promethazine (Painstop) for premedication and analgesia for myringotomy in children.

This prospective double-blinded study compared the analgesic effectiveness and incidence of complications of a compound preparation Painstop (Paedpharm Pty Ltd) containing paracetamol 12 mg, codeine 0.5 mg and promethazine 0.65 mg per 1.0 ml, dosage 1.0 ml/kg, with paracetamol 20 mg/kg. Ninety-five children aged 1 to 12 years, ASA 1-2, scheduled for myringotomy and drain tuber insertion as a day procedure were randomized to receive Painstop or paracetamol 30 to 60 minutes prior to surgery. Preoperative drowsiness and complications on induction and postoperative sedation, pain and times to achieve goals were recorded. The groups were comparable for age, gender, weight, anaesthetic technique and duration of surgery. Times to eye opening (P = 0.05) and first oral intake (P = 0.006) were significantly longer in the Painstop group. There was, however, no difference in times to discharge. Late sedation was more common in the Painstop group (P = 0.03). Pain scores were low and similar in both groups and the need for additional analgesia was uncommon.

Acetaminophen↗

PubMed2005

Paracetamol use in musculoskeletal pain: an audit of use and patient perceptions of paracetamol as an effective analgesic.

Musculoskeletal pain is a complex problem with often very detrimental consequences which affects a high proportion of the general population. Health care professionals, when prescribing for musculoskeletal pain, often overlook simple analgesia. Patient perceptions of analgesia may vary to those of health care professionals, and in part affect the use of simple analgesia for musculoskeletal pain. This paper describes an audit of paracetamol use and patient perceptions of paracetamol as an effective analgesic agent, in 113 patients attending a musculoskeletal pain outpatient clinic in a university teaching hospital. The audit has helped prompt the development of a multi-disciplinary strategy to achieve optimum management.

Journal Article↗

PubMed1988

Complete separation of urinary metabolites of paracetamol and substituted paracetamols by reversed-phase ion-pair high-performance liquid chromatography.

A reversed-phase high-performance liquid chromatographic procedure has been developed for the separation of thirteen urinary metabolites of the analgesic drug paracetamol. The method involved the use of radially compressed columns packed with octadecylsilica with a particle diameter of 5 micron. Metabolites were chromatographed by linear gradient elution using an ion-pair solvent system composed of tetrabutylammonium hydroxide and Tris buffered to pH 5.0 with phosphoric acid, and acetonitrile as the organic solvent. Analyses can be performed at the rate of three per hour. This method enables the direct identification of sulphate and glucuronide conjugates of 3-thiomethylparacetamol and 3-thiomethylparacetamol sulphoxide which have only previously been detected following enzyme hydrolysis of urine samples. The application of this fully optimised separation to the study of the metabolism of substituted paracetamols is also discussed.

Acetaminophen↗

PubMed1979

Effect of paracetamol on amylobarbitone hydroxylation in man: a gas chromatographic method for simultaneous estimation of underivatized paracetamol and barbiturates.

A rapid and specific technique for the simultaneous gas chromatographic estimation of underivatized paracetamol and barbiturates using an alkali flame ionization detector is described which is suitable for both forensic and pharmacokinetic investigations. An improved method for estimation of 3-hydroxyamylobarbitone is also detailed. These techniques were used in an investigation of the effects of oral administration of 1 g paracetamol 8 hourly on the formation of 3-hydroxyamylobarbitone from a single oral dose of 200 mg sodium amylobarbitone. No significant changes were found in the plasma concentrations and total body clearance of amylobarbitone nor was there any alteration in the urinary elimination of 3-hydroxyamylobarbitone.

Acetaminophen↗

PubMed1983

A double-blind comparison in general practice of a combination tablet containing orphenadrine citrate and paracetamol ('Norgesic') with paracetamol alone.

A double-blind trial of a combination tablet containing orphenadrine and paracetamol, 'Norgesic', was carried out to assess the value of this product compared with paracetamol alone. All three symptomatic parameters which were measured: pain, spasm and impaired activity, showed a significantly quicker recovery when the combination product was used. Further studies are necessary to evaluate the combination product against orphenadrine citrate alone.

Acetaminophen↗

PubMed1978

[Investigations concerning temperature and serum concentrations of paracetamol in febrile infants following rectal application of paracetamol (author's transl)].

If paracetamol as an antipyretic is applied as suppository, absorption is much slower than after oral application. With the same dosage serum concentrations are lower after rectal application than after oral. It is necessary to give nearly twice the dose using rectal rather than oral application to reach the same antipyretic effect. The duration of efficacy is longer after rectal application because serum concentrations decline slower.

Acetaminophen↗

PubMed1994

[Paracetamol poisoning in infancy--is paracetamol safe?].

A 5-month-old baby was observed. At the severely ill infant accidental misadministration of Paracetamol was verified. After surveying the literature the authors discussed the possible pathobiochemical ways and draw attention to the dangers of the misuse of antipyretics.

Acetaminophen↗

PubMed1975