Nail biting is a common behavioral problem. While there are established behavioral interventions for management,
they are of modest efficacy, and there is minimal evidence for effective pharmacotherapy. This study investigated
the role of N-acetylcysteine (NAC) a potent glutathione and glutamate modulator for the treatment of pathological nail
biting in children and adolescents. This pilot randomized, double-blind, placebo-controlled clinical trial of NAC
(800mg/day) or placebo enrolled 42 children and adolescents with chronic nail biting. Nail length was the objective outcome.
Evaluations were carried out three times; before treatment, one month after enrollment in the study, and two
months after enrollment. The duration (chronicity) of nail biting in the NAC and placebo groups was 3.63(2.45) and
5.09(3.74) years (P=0.14). The mean nail length gradually increased in both the NAC and placebo groups during this trial.
There was a statistically significant difference between the two groups regarding increased nail length after the first month
of trial [(5.21(5.75) and 1.18(3.02) millimeters], however no difference after two months was observed. Two patients in
the NAC group discontinued medication due to adverse events. One patient experienced headache, agitation, and social
withdrawal, and another patient expressed severe aggression after taking medication and was withdrawn from the study.
This study supports the hypothesis that NAC decreases nail biting behavior in children and adolescents over the short
term. NAC is relatively well tolerated and severe adverse effects are rare. However, there was a high rate of dropout. Further
studies with longer durations that build on these preliminary data are recommended. This study is registered at the
Iranian Registry of Clinical Trials (Irct registration number: IRCT201103023930N3).