Intervention: Serine
Trial phase: Phase 4
SMILES: NC(CO)C(=O)O
Sex: All
Age groups: Child
Healthy volunteers: No
Participants at risk: 39
Group description: 24 subjects age 8-17 years with moderate to severe TS (TTS > 22) will be enrolled in this treatment arm. They will receive D-serine for 6-weeks of the study, during which time drug dose may gradually be increased as needed. At 6-weeks participants will taper off drug.

D-serine: The dosage schedule will be flexible with a maximum dose for each subject being 30 mg/kg/day. In any individual subject, dose escalation may proceed more slowly, or the dose may be reduced if necessary. At the 4 week visit, if an additional dosage increase is prescribed by Dr Singer, the pharmacy will provide an additional vial of capsules labeled as study drug, but containing either 250 or 500 mg tablets of D-serine or placebo; capsule content to be determined by patient's weight. No changes in dosage will be made during the final week of treatment.
Eligibility criteria:
Inclusion Criteria:

Tourette syndrome (criteria based on the TS Classification Study Group), which includes onset before 18 years, multiple involuntary motor tics, one or more vocal tics, a waxing and waning course, the gradual replacement of old symptoms with new ones, the presence of tics for more than one year, the absence of other medical explanations (effects of a substance (e.g., stimulants) or a general medical condition for tics, and observation of tics by a reliable examiner) or Chronic Motor Tic disorder (criteria similar to Tourette syndrome except for the absence of vocal tics)
Age 8-17 years, either gender
Observable tics, achieving a minimum score of > 22 on the Total Tic score of Yale Global Tic Severity Scale (YGTSS)
Tic symptoms severe enough to warrant therapy (e.g., causing psycho-social or physical difficulty)
Written informed consent provided by the patient's parent (or legal guardian) and assent provided by the patient consistent with Institutional Review Board requirements
Ability and willingness to comply with study protocol requirements
Women of childbearing potential must be using a medically acceptable contraceptive method. Acceptable methods of birth control are limited to: Intra-Uterine Device (IUD), oral, implantable, injectable contraceptives and estrogen patch, double barrier method (spermicide+diaphragm), or abstinence
Baseline weight of at least 33 kilograms
Tic-suppressing drug naive, or currently not on treatment for TS (off medications for at least three weeks), or if, in the judgment of the PI, they are not adequately managed using current therapy (prescribed for greater than one month) and are willing to maintain a constant dose throughout the protocol.

Exclusion Criteria:

Secondary tics
Significant medical illness (metabolic, endocrine, cardiac, hematological, gastrointestinal, pulmonary, epilepsy)
Current major depression
generalized anxiety disorder
separation anxiety disorder
psychotic symptoms (based on clinical evaluation and the results of the CY-BOCS, CDI-S, and MASC evaluations)
pervasive developmental disorder
autism
mental retardation (I.Q. less than 70)
anorexia/bulimia, or substance abuse
Any other conditions that in the opinion of the Investigators would interfere with the evaluation of the results or constitute a health hazard for the patient
Pregnancy
Hypersensitivity to D-serine or riluzole
Abnormal laboratory values on screening laboratory testing if clinically significant at the Principal Investigator's discretion.

Subjects with co-morbid ADHD, obsessive compulsive disorder (OCD), and conduct disorder will not be excluded as long as these diagnoses are not the subject's primary problem