Use of N-Acetylcysteine (NAC) in Fingernail Biting

Inclusion Criteria: * 1\. Age 18-30 2. Fingernail biter for ≥5 years Exclusion Criteria: * 1\. Any unstable major DSM-IV Axis I psychiatric disorder in the past month (e.g., psychotic disorders, bipolar disorder, major depressive disorder) 2. Current substance dependence in the past month 3. Current unstable major medical disorder or medical condition in the past 6 months (e.g., renal impairment) 4. Current pregnancy or breastfeeding 5. Current seizure disorder or asthma, due to possible elevated risk for adverse effects with NAC in individuals with these conditions 6. Known hypersensitivity to NAC 7. Use of carbamazepine or nitroglycerin (or any other medication deemed to be hazardous if taken with NAC) within 14 days of study participation