There are medications for sure that mess up cortisol testing. Typically though they don't mess with blood levels but usually free cortisol testing which is urine and saliva so that cortisol binding globulin is changed.
I found this:
"Question: Are there any drugs that interfere with cortisone metabolism or measurement that I shouldn't take while I am tapering off of replacement medication?
Answer: A number of drugs interfere with cortisol metabolism including anti-seizure medications and anti-fungal medications. Growth hormone therapy may affect cortisol replacement. The measurement of cortisol in a blood test is affected by many medications, especially estrogen which can cause the blood level to read as higher than it really is. A urine test for cortisol will not be affected by estrogen. This does not necessarily mean that patients should not take these medications during recovery, only that the patient and the patient's doctor need to be informed. "
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