Adolescents who show both obsessive-compulsive behaviour and pronounced, labile mood present a treatment challenge: the usual recommendation of lithium, valproate or other mood stabilisers can bring metabolic and cognitive costs, yet withholding them risks an antidepressant-induced switch to mania. A 17-year-old boy illustrates an alternative course. He had childhood ADHD and long-standing checking rituals; while at boarding school overseas he developed intense rumination, wide mood swings and passive suicidal thoughts. Low-dose risperidone failed to help. Rather than add a conventional mood stabiliser, we introduced an oral glutamatergic stack aimed at reproducing ketamine-like plasticity: dextromethorphan for NMDA antagonism, piracetam to potentiate AMPA transmission and 10 mg fluoxetine nightly to slow dextromethorphan metabolism. Within weeks mood steadied, intrusive thoughts abated and function at school improved. The patient remained well without therapeutic-dose mood stabilisers. This experience suggests that carefully layered oral glutamatergic therapy may, in selected adolescents, deliver both anti-obsessional and mood-stabilising benefits while avoiding the burdens of traditional agents.