This is a Phase 3 study to evaluate the efficacy, safety, and tolerability of crinecerfont versus placebo administered for 24 weeks in approximately 165 adult participants with classic CAH due to 21-hydroxylase deficiency. The study consists of a 24-week randomized, double-blind, placebo-controlled period, followed by 1 year of active treatment with crinecerfont. Subsequently, participants may elect to participate in the open-label extension (OLE) period. The duration of participation in the study is approximately 20 months for the core study and will be a variable amount of time per participant for the OLE (estimated to be approximately 3 years).

Inclusion Criteria:

1. Be willing and able to adhere to the study procedures, including all requirements at the study center and return for the follow-up visit.
2. Have a medically confirmed diagnosis of classic CAH due to 21-hydroxylase deficiency.
3. Be on a stable steroid regimen.
4. Participants of childbearing potential must agree to use an acceptable method of contraception during the study.

Exclusion Criteria:

1. Have a diagnosis of any of the other known forms of classic CAH.
2. Have a history of bilateral adrenalectomy, hypopituitarism, or other condition requiring chronic glucocorticoid therapy.
3. Have a clinically significant unstable medical condition or chronic disease other than CAH.
4. Have a history of cancer unless considered cured.
5. Are pregnant.
6. Have a known history of clinically significant arrhythmia or abnormalities on ECG.
7. Have a known hypersensitivity to any corticotropin releasing hormone receptor antagonists.
8. Have received any other investigational drug within 30 days before initial screening or plan to use an investigational drug (other than the study drug) during the study.
9. Have current substance dependence, or current substance (drug) or alcohol abuse.
10. Have had a blood loss ≥550 mL or donated blood or blood products within 8 weeks prior to the study.