This study is a double-blind, double-dummy, two-way cross-over, randomised, Phase II study to be conducted at approximately 6 investigational sites in 2 countries. The study will compare the efficacy, safety and tolerability of twice daily Chronocort, a modified-release hydrocortisone, with once daily Plenadren, a modified-release hydrocortisone, over a treatment period of up to 2 months in participants aged 18 years and over, diagnosed with primary Adrenal Insufficiency (AI).

The 4-month study period will comprise a Screening Period of up to 4 weeks (during which participants will receive usual standard of care \[SoC\] treatment), two 4-week cross-over periods (Treatment Periods 1 and 2), and a 4-week Follow-up Period. Participants will be randomly assigned on a 1:1 basis to either Chronocort in Treatment Period 1 and Plenadren in Treatment Period 2 (Treatment Sequence I), or Plenadren in Treatment Period 1 and Chronocort in Treatment Period 2 (Treatment Sequence II). The total daily dose of Chronocort or Plenadren will be 25 mg according to international guidelines and the Plenadren Summary of Product Characteristics (SmPC). Plenadren 25 mg will be taken once daily in the morning on waking (typically between 06:00 and 08:00 hours). Chronocort 10 mg will be taken in the morning on waking (typically between 06:00 and 08:00 hours) and Chronocort 15 mg will be taken at night just prior to going to bed (typically between 22:00 hours and midnight). Placebo capsules and tablets will be provided to each participant to mask the treatment being received and maintain the study blinding. The first dose of study drug will be taken in the evening of Day 1 and the morning dose on Day 29 will be the last study drug dose in Treatment Period 1. The first dose of study drug in Treatment Period 2 will be taken in the evening of Day 29 and the morning dose on Day 57 will be the last study drug dose in Treatment Period 2 and the end of the dosing periods. The dose will not be changed during the study, but participants will receive stress dosing rules and an emergency treatment pack. Use of stress doses and reasons for use will be collected in an electronic participant diary. If a stress dose is taken within 48 hours before any clinic visit, then the visit should be delayed until the participant has had a 48-hour period without use of any stress doses.