Abstracts

Rationale: Lamotrigine is an antiepileptic drug (AED) frequently used in partial, generalized and photosensitive epilepsies, as well as in pregnancy. Phencyclidine (PCP) is a drug of abuse that is also often an adulterant in marijuana and methamphetamines. The ubiquitous ELISA urine drug screens often obtained due to abnormal periictal behavior has led to patients on lamotrigine being labeled as drug users. Methods: On retrospective review of our electronic database over 5 years, we identified patients on lamotrigine therapy. We then compared the results of those with positive initial urine drug screens to those whose confirmatory Western Blot tests came back positive several days later. We compared false positive results with lamotrigine serum levels and doses.Results: Of the patients on lamotrigine therapy who had urine drug screens, nearly half tested positive for PCP on the preliminary ELISA testing, but negative on Western Blot. Most of the patients who eventually took lamotrigine, and who initially tested negative for PCP were not on lamotrigine at the time of drug screening. Lamotrigine levels in positive samples were about twice as high as levels in negative samples, suggesting that lamotrigine has a dose and concentration-dependent effect on false PCP positivity.Conclusions: Caregivers should be cautious of initial PCP positives on screening in any patients taking lamotrigine, particularly those with higher doses or serum levels. Patients should never be labeled as PCP abusers prior to confirmatory testing. Animal studies suggest that lamotrigine suppresses PCP-induced cortico-limbo-thalamic relative cerebral blood flow changes, and prevents NMDA receptor antagonists psychotomimetic effects in animals and humans. We speculate that lamotrigine may have a role in treating PCP toxicity and side effects. It also has potential in treating cognitive symptoms in other psychotic conditions such as schizophrenia and mania.