The study will be looking at the cortisol profiles of hypocortisolaemic patients following 100mg injections of two hydrocortisone preparations (Solu-Cortef® & Hydrocortisone 100mg/ml). The investigators plan to use two methods of intramuscular injection to administer the preparations, one using a 1inch orange needle into the deltoid muscle and the other using a 1.25inch blue needle into the thigh muscle.

Inclusion Criteria:

1. Age 18-70 inclusive
2. Written informed consent provided, prior to any study related assessments/procedure being conducted.
3. Men & Women with a BMI between 18-30kg/m2
4. Addison's disease or Bilateral Adrenalectomised patients with pre-hydrocortisone cortisol level below 100nmol/L and ACTH greater than 50ng/L on screening visit
5. All patients must be stabilised on hydrocortisone with no change in dosage for 6 months, other than transient increases for concurrent illness.
6. Able to self-inject into deltoid and thigh muscles following teaching at recruitment.
7. Female patients of child-bearing potential must be willing to use an acceptable method of birth control/abstinence from the time consent is signed until 6 weeks after treatment is discontinued. Acceptable methods include: physical barrier (male or female condom, contraceptive sponges, diaphragms and cervical caps),-contraceptive pill or patch, spermicidal method alongside a physical barrier or an intrauterine device (IUD). Abstinence is also acceptable if it falls in line with the patient's usual lifestyle however it must be complete abstinence and not either; periodic, ovulation timed, symptothermal or withdrawal based. Those patients that utilise hormonal contraceptives must have used the same method for at least three months before additional barrier contraception (as described above). Patients of non-child-bearing potential are defined as having 12 month amenorrhoea or are surgically sterile.

Exclusion Criteria:

1. Patient on oestrogen based or mixed oral contraceptives unless willing to use alternate effective method of contraception
2. Patient on any forms of oral steroids other than hydrocortisone.
3. Any patient with secondary adrenal failure
4. Patients with a diagnosis of any disease or condition listed in Hydrocortisone 100mg/ml and Solu-Cortef®'s SmPC as being contraindicated or precautionary for use
5. Patient with concurrent illness in the week preceding screening/study visit.
6. Patient must not have had an adrenal crisis in the week before screening
7. Patient with Nelson's syndrome.
8. Participating in another IMP investigation
9. Patient who is unable or unwilling to comply with the protocol.
10. Patient taking any medications/substances that are known to interact with hydrocortisone e.g. CYP3A4 inhibitors
11. Pregnant or breastfeeding patients
12. Patient has any other disease or condition that, in the opinion of the investigator, might compromise patient safety or interfere with the results of the trial