Effect of D-cycloserine on a Short Imagery Rescripting Intervention for Subclinical PTSD

Inclusion Criteria: * Adult outpatients ≥ 18 years of age, who have experienced a traumatic event such as sexual assault, physical assualt, a serious accident, or other event where they feared for their life or their safety, at least 3 months prior to intake, with a primary subclinical psychiatric diagnosis of post-traumatic stress disorder (PTSD) as measured by the CAPS-5 (structured clinical interview to assess for PTSD according to the DSM-5). Eligible participants will have a CAPS-5 score of mild or moderate. * Physical examination and laboratory findings within normal limits, as determined by the study nurse. * Willingness and ability to participate in the informed consent process and comply with the requirements of the study protocol. * Potential subjects must have sufficient command of the English language. Exclusion Criteria: * A lifetime history of bipolar disorder, schizophrenia, psychosis, delusional disorders or organic brain syndrome; past history of reported or current seizures; use of Isoniazid (a particular antibiotic); cognitive dysfunction that can interfere with capacity to engage in therapy; * A history of substance or alcohol dependence (other than nicotine) in the last 6 months (or otherwise unable to commit to refraining from alcohol use during the acute period of study participation). The acute period of study participation is defined as during their visit and 24 hours before and after their visit. * Patients with significant suicidal ideation or who have enacted suicidal behaviors within 6 months prior to intake will be excluded from study participation and referred for appropriate clinical intervention. * Patients cannot be taking psychotropic medication during the study period. They have to be off psychotropic medication for three weeks. * Participating in ongoing exposure-based psychotherapy for PTSD or psychodynamic therapy focusing on exploring specific, dynamic causes of the traumatic symptomatology and providing management skills. General supportive therapy initiated \> 3 months prior to study is acceptable. * Significant personality dysfunction likely to interfere with study participation. For example, overly aggressive behavior or disruptive behavior that might jeopardize safety of the staff or impairs providing the treatment. * Serious medical illness or instability for which hospitalization may be likely within the next year. For example, if people are currently in a treatment for cancer, or people that are waiting for organ donation. This decision would be determined by our medical staff during the eligibility screen. * Patients with a current or past history of epilepsy or seizures. * Patients who have experienced any cardiac event. Patients with clinically significant abnormalities in vital signs (e.g., systolic blood pressure \>150 mm Hg or diastolic blood pressure \>100 mm Hg) at screening will be excluded from further study participation and referred for appropriate clinical management. * Pregnant women, lactating women, women who are breastfeeding and women of childbearing potential who are not using medically accepted forms of contraception (e.g., IUD, oral contraceptives, or implanted progesterone rods stabilized for at least 3 months). * Patients with a history of head trauma causing loss of consciousness, or ongoing cognitive impairment. * Patients who experienced multiple events of interpersonal trauma prior to the age of 14.