Trichotillomania (TTM) is an impulse control disorder in which patients feel that they can relieve tension by pulling out hair from different parts of their bodies. While sharing some features with obsessive compulsive disorder, DSM-IV-TR does not officially classify it as a type of OCD. The pharmacological treatment of trichotillomania has often frustrated patients and clinicians alike. Clomipramine has outperformed place- bo, but its benefits appear to diminish substantially after discontinuation and SSRIs have failed to beat placebo across four trials (Bloch MH et al., Biol Psychiatry 2007;62:839-846). A recent study compared an amino acid (N-acetyl- cysteine, or NAC) to placebo. Why NAC? Because NAC is converted to cysteine, which reduces synaptic release of gluta- mate. Some research has implied that repetitive behaviors like TTM my be caused by excessive glutamate, providing a biochemical rationale for a trial of NAC for TTM. Of the 50 enrolled patients, most had at least one comorbid condi- tion, including depression (28%), an anx- iety disorder (28%), or another impulse control disorder such as compulsive skin picking (36%). About half were taking other psychotropic medications, and maintained their dose of these drugs dur- ing the study. Participants took 1200 mg/day of NAC for six weeks (or match- ing placebo), then increased dosage to 2400 mg daily for the remaining six weeks. Patients on NAC outperformed placebo patients on all trichotillomania measures by a quite large effect size. 44% of NAC patients experienced a 50% or greater drop in trichotillomania symptom rating scores compared to zero patients on placebo. NAC did not improve depression or anxiety scores more than placebo. Remarkably, no patients on NAC reported any side effects (Grant JE et al., Arch Gen Psychiatry 2009;66:756-763), though NAC may worsen asthma.