Inclusion Criteria:

• Male and female subjects 18 to 55 years old, inclusive.
• Has a documented historical diagnosis of classic CAH due to 21-hydroxylase deficiency based on genetic mutation in CYP21A2 and/or elevated 17-OHP.
• Has A4 > 1.5 x ULN and ACTH > 2 x ULN at both screening and Week 4 (measured before any AM GC dose).
• Has been on a stable dose of GC replacement ≥ 15 mg/day and ≤ 50 mg/day in HCe that does not vary in total daily dose from day to day for ≥ 3 months before screening without any evidence of non-adherence to the GC regimen during this period (stress dosing is allowed).
• For subjects with the salt-wasting form of CAH, subject has been on a stable dose of mineralocorticoid replacement for ≥ 3 months before screening.
• Agrees to follow contraception guidelines. Male subjects must also agree to refrain from donating sperm throughout the treatment period and for 90 days after the last dose of study drug.
• Is able to understand all study procedures and risks involved and provides written informed consent indicating willingness to comply with all aspects of the protocol

Exclusion Criteria:

• Has a known or suspected diagnosis of any other known form of classic CAH (not due to 21-hydroxylase deficiency).
• Has a history that includes bilateral adrenalectomy or hypopituitarism.
• Has a history of allergy or hypersensitivity to tildacerfont or any other CRF1 receptor antagonist. 
• Current treatment with dexamethasone as GC therapy for CAH:
  • Prior treatment with dexamethasone is allowed as long as the transition to an alternative GC regimen (e.g., HC, prednisone, or prednisolone) has resulted in a stable dose of GC replacement for ≥ 3 months before screening.
• Is not adherent to GC or study drug dosing regimen during the Run-in Period (defined as taking 12 for either depression or anxiety at screening or Week 6.
• Shows clinical signs or symptoms of adrenal insufficiency.
• Has had a clinically significant unstable medical condition, medically significant illness, or chronic disease occurring within 30 days of screening, including but not limited to:
  • An ongoing malignancy or < 3 years of remission history from any malignancy, other than successfully treated localized skin cancer;
  • Presence of clinically significant renal disease, as evidenced by an estimated glomerular filtration rate (eGFR) of < 60 mL/min/1.73 m^2;
  • Current or chronic history of liver disease (including nonalcoholic steatohepatitis and female subjects with a history of intrahepatic cholestasis of pregnancy) or known hepatic or biliary abnormalities (with the exception of Gilbert’s syndrome or asymptomatic gallstones);
  • History of alcohol or substance abuse within the last year, or any significant history of alcohol or substance abuse that would likely prevent the subject from reliably participating in the study, based on the opinion of the Investigator;
  • Active hepatitis B, hepatitis C, or human immunodeficiency virus (HIV) at screening.
• Had or has a clinically significant psychiatric disorder, including the following:
  • Evidence of major depressive episode, bipolar disorder, schizophrenia, schizoaffective disorder, major depressive disorder with psychotic features, or any other psychotic disorder within the preceding 6 months;
  • Increased risk of suicide based on the Investigator’s judgment or the results of the Columbia–Suicide Severity Rating Scale (C-SSRS) conducted at screening and Week 6 (e.g., C-SSRS Type 3, 4, or 5 ideation within the past 6 months or any suicidal behavior within the past 12 months);
  • HADS score > 12 for either depression or anxiety at screening or Week 6
• Has clinically significant abnormal electrocardiogram (ECG) or clinical laboratory results. Abnormal results that must be reviewed and discussed with the Medical Monitor to determine eligibility for this study include but are not limited to:
  • Any clinically meaningful abnormal ECG results, including Fridericia-corrected QT interval (QTcF) > 450 ms for male participants or > 470 ms for female participants;
  • Alanine aminotransferase (ALT) > 2 x ULN;
  • Total bilirubin > 1.5 x ULN (isolated total bilirubin > 1.5 x ULN is acceptable if bilirubin is fractionated and direct bilirubin is 5 x ULN;
  • Total bile acids > 5 x ULN.
• Routinely works overnight shifts.
• Subjects with travel plans/work schedules that result in significant and frequent changes in time zones (> 2 hours) will require Medical Monitor approval for enrollment.
• Females who are pregnant or nursing
• Use of any other investigational drug from 30 days or 5 half-lives (whichever is longer) before screening to the end of the study.
• Use of rosiglitazone, aromatase inhibitors, testosterone, or growth hormones from 30 days or 5 half-lives (whichever is longer) before Day 1 to the end of the study:
  • Use of strong inhibitors and/or inducers of cytochrome P450 3A4 (CYP3A4) (with the exception of GCs and birth control) and medications metabolized by CYP3A4, 2B6, 2C8, 2C9, or 2C19, especially those that are sensitive substrates or substrates with narrow therapeutic ranges, should be discussed on a case-by-case basis with the Medical Monitor to determine whether the medication should be discontinued or may be continued with caution. If washout is feasible, then the medication should be withdrawn at least 30 days or 5 half-lives (whichever is longer) before Day 1.
• Use of any anticoagulants or antiplatelet therapies within 30 days before screening.
• Has a history of an active bleeding disorder.
• Donation of blood from 60 days before Day 1 to the end of the study; or donation of platelets, white blood cells, or plasma from 15 days before Day 1 to the end of the study