To conduct a pilot randomized placebo-controlled feasibility trial of N-acetylcysteine (NAC) in onychophagia, preliminarily assessing the tolerability and effects of this pharmacotherapy on young adults who bite their fingernails. It is proposed that NAC will help decrease fingernail biting.

This is a proof-of-concept pilot placebo-controlled trial of N-acetylcysteine (NAC) in young people who bite fingernails. The primary interest is in assessing the feasibility of conducting this line of research, while preliminarily exploring potential effects on nail biting, as measured by nail length.

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