Introduction. It is important to know which clinical situations prevent ketamine from working. Case Report. We present the case of the psychiatric inpatient who was admitted to our emergency department after ingesting a toxic dose of lamotrigine, unknown at that time. On admission, she was clearly in distress, displaying extreme agitation and violent ataxic movements. We opted to achieve sedation using intravenous ketamine boluses. Unexpectedly, after being injected with a total of 250 mg ketamine, our patient displayed no signs of dissociative anaesthesia. Discussion. There was no apparent reason for why ketamine failed, but an interaction between lamotrigine and ketamine was suspected. A literature search was performed. Very few articles describe interactions between lamotrigine and ketamine. Experimental studies, however, demonstrate how lamotrigine attenuates the neuropsychiatric effects of ketamine. Ketamine is classically described as an NMDA antagonist. Ketamine's dissociative effects, however, are thought to be mediated by increased glutamate release via a pathway not dependent on NMDA receptors. Lamotrigine, on the other hand, is known to reduce cortical glutamate release. Conclusion. Lamotrigine reduces the glutamate release needed to mediate ketamine's dissociative anaesthesia. This is important knowledge for anaesthesiologists in the emergency room where ketamine is often administered to unstable patients.