Drug DescriptionParticipants with IBS (all subtypes) and with no exclusionary comorbid psychiatric or medical disorders will be enrolled in the study. This study will involve a randomized waitlist control design to investigate the rapid and sustained effects of TRP-8802 following two experimental sessions in which an oral dose of TRP-8802 is administered to participants with IBS. The study will include clinician and participant ratings of depression and anxiety pre- and post-drug-session, monitor and participant ratings of subjective drug effects during and after each drug session. This study comprises approximately a 28-day screening period (Days 28 to 1). After screening and enrollment, participants will be randomized to an immediate treatment group or a delayed treatment group ("waitlist control" condition). Participants in the immediate treatment group will proceed directly into three weeks of baseline and preparation (Days 1 to 18), a 2-dose administration period (Days 22 and 37), integration (Days 23, 30, 38, and 45), the End of Therapy (EOT) visit (Day 52). Participants in the delayed treatment group will wait 8 weeks after enrollment before beginning the study interventions and neuroimaging assessments. As a safety precaution, participants in the delayed treatment group will be assessed weekly via telephone calls or in-person visits during the wait period (i.e., telephone assessments during post-randomization weeks 1, 2, 3, 4, 5, 6, and 7; in-person assessment during post-randomization week 8) to assess suicide risk to determine if intervention is warranted. During week 8, IBS symptoms will also be assessed. At the end of the delay period, all participants in the delayed treatment group will complete the same intervention as the participants in the immediate treatment group. Validated and commonly used assessment tools will be used to evaluate symptoms at baseline and repeatedly after each session. The weekly average of worst daily pain score and weekly stool frequency and consistency for the 7 days immediately prior to EOT visit will be assessed for change from baseline and at the 3-, 6 , and 12- month follow-up visits (Days 120, 240, 365). 
