title: Variation in dose and plasma level of lamotrigine in patients discharged from a mental health trust.
abstract: BACKGROUND
  The objectives of this study were to investigate the dose of lamotrigine when prescribed with an enzyme inhibitor or enzyme inducer in patients discharged from a mental health trust and to determine the corresponding lamotrigine plasma concentrations and the factors that may affect these.
  
  
  METHODS
  All patients discharged on lamotrigine between October 2007 and September 2012 were identified using the pharmacy dispensing database. We recorded demographic details, lamotrigine dose and plasma levels and coprescribed medication.
  
  
  RESULTS
  During the designated period, 187 patients were discharged on lamotrigine of whom 117 had their plasma levels recorded. The mean lamotrigine daily dose was 226.1 mg (range 12.5-800 mg) and the mean plasma level 5.9 mg/l (range 0.8-18.1 mg/l). Gender, ethnicity, diagnosis and smoking status had no significant effect on dose or plasma levels. Patients taking an enzyme-inducing drug (n= 6) had significantly lower plasma levels [mean (SD) 3.40 (1.54) mg/l] than those not taking enzyme inducers [n= 111; 6.03 (3.13) mg/l;p= 0.043]. Patients taking an enzyme-inhibiting drug (n= 23) had significantly higher levels [7.47 (3.99) mg/l] than those not taking an inhibitor [n= 94; 5.52 (2.75) mg/l;p= 0.035]. No significant difference was found between the doses of lamotrigine in patients taking an enzyme inhibitor and those not taking one (p= 0.376). No significant difference was found between the doses of lamotrigine in patients taking an enzyme-inducing drug and those not taking any (p= 0.574).
  
  
  CONCLUSIONS
  Current dosing recommendations indicate that lamotrigine doses should be halved in individuals taking enzyme inhibitors and doubled in those on enzyme inducers. In our survey these recommendations were rarely followed with the consequence that patients received too high or too low a dose of lamotrigine, respectively.
authors.affiliation: Clinical Pharmacist, South London and Maudsley NHS Foundation Trust, Pharmacy Department, Maudsley Hospital, Denmark Hill, London SE5 8AZ, UK.
authors.forename: Petrina
authors.initials: P
authors.lastname: Douglas-Hall
authors.affiliation: Research Pharmacist, South London and Maudsley NHS Foundation Trust, Pharmacy Department, Maudsley Hospital, London, UK.
authors.forename: Olubanke
authors.initials: O
authors.lastname: Dzahini
authors.affiliation: Consultant Psychiatrist, South London and Maudsley NHS Foundation Trust, National Psychosis Service, Maudsley Hospital, London, UK.
authors.forename: Fiona
authors.initials: F
authors.lastname: Gaughran
authors.affiliation: Pharmacist, Hounslow East Pharmacy, Middlesex, UK.
authors.forename: Ahmed
authors.initials: A
authors.lastname: Bile
authors.affiliation: Director of Pharmacy and Pathology, Professor of Psychopharmacology, South London and Maudsley NHS Foundation Trust, Pharmacy Department, Maudsley Hospital, London, UK Institute of Pharmaceutical Science, King's College, London, London, UK.
authors.forename: David
authors.initials: D
authors.lastname: Taylor
keywords: drug interactions; lamotrigine; therapeutic drug monitoring