Medications such as anti-seizure drugs, estrogen, androgens and certain synthetic steroids sometimes interfere with cortisol blood tests, according to MedlinePlus. Steroids that are potentially problematic are glucocorticoids, including hydrocortisone, prednisone and prednisolone.
Doctors sometimes tell patients to stop taking medicines prior to cortisol blood tests, but the length of time varies by drug, states WebMD. To ensure the doctor's ability to diagnose any possible problems correctly, the patient needs to mention all the over-the-counter and prescription drugs he takes.
The cortisol blood test measures the amount of cortisol, a hormone, in the blood, MedlinePlus says. Abnormal levels sometimes indicate disorders of the pituitary or adrenal glands, including tumors, Cushing disease or hypopituitarism.