I was taking prednisone 20mg twice daily for 5 days. I was wondering how long before it is out of my system? I have a drug test coming up and was told that the prednisone can cause a false-negative.
It can be difficult to give an exact time-frame for how long prednisone stays in your system as several factors can influence this, including:
Nevertheless, we can get a fairly accurate estimate based on the known half-life of prednisone, which is the time it takes to metabolize and eliminate 50% of the drug from the body.
Prednisone has been extensively studied and has a reported half-life between two and three hours.
For most individuals, it takes around 5 to 6 half-lives for a drug to be considered completely eliminated (i.e. out of your system).
Therefore, being conservative and assuming a half-life of three hours, you could expect prednisone to be out of your system around 18 hours after your last dose (6 half-lives X 3 hours).
Again, this may vary slightly from person to person, but you can reasonably expect that prednisone will be completely eliminated within a fairly well-defined range of 12 to 18 hours.
As mentioned at the beginning of this article, there are several factors which can influence how long prednisone stays in your system.
Perhaps the most significant variable is liver function since it is responsible for prednisone metabolism.
If you have decreased liver function, be it from disease or other factors (such as old age), you can expect for prednisone to be more slowly eliminated.
Similarly, if you take drugs that decrease the activity of certain liver metabolizing enzymes, like CYP3A4, it could take longer for prednisone to be metabolized. Strong inhibitors of CYP3A4 include:
There are many other factors that could influence prednisone metabolism (e.g. age, kidney function, length of time taking the drug), which is why it is so difficult to give an exact determination on how long it will be present in your system. Overall though, we can expect 12 to 18 hours to be a fairly accurate estimate.
It is important to note that even though has a fairly short half-life and is eliminated relatively quickly when compared to many other drugs, it's biological action last much longer. In fact, most sources state that the effects of prednisone last between 18 to 36 hours per dose.
This long duration of action is due to the fact that the drug is highly protein bound in the plasma (specifically to albumin and transcortin) and because it is extensively distributed to various organs in the body (e.g. kidneys, liver).
It also is important to point out that prednisone itself is not active, but is metabolized in the liver to the active metabolite prednisolone. Prednisolone has a similar half-life to prednisone, around two to three hours.
Prednisone has not been reported to cause false-positives for the most commonly screened drugs on urinalysis tests, which are:
Nevertheless, due to its similarity in structure to other steroids, it could feasibly cause false-positive readings for anabolic steroids or similar drugs.
It has been reported that prednisone (and other immunosuppressive drugs) can cause false-negatives on certain diagnostic tests, like tuberculin (TB) skin and allergy tests since it can reduce immune response.
It is important to discuss your medication therapy with your doctor so they can advise you on how long before testing you need to stop prednisone (if necessary).