Brief Answer:
Approximately 36 - 54 hours.

Detailed Answer:
According to the pharmaceutical literature, the "half-life" for oral Decadron (dexamethasone) is about 3 hours. That is the length of time it takes for half of the medicine to be left in your body. However, some sources say that the length of activity can be between 36 and 54 hours.

I am copy/pasting some information about this below.

If you find that the rash returns in the next couple of days, then I would advise that you see your doctor as you may need a longer course of corticosteroid medication. The shorter you are on these the better, but 1 week of a tapering dose is generally not long enough to cause significant problems. However, this one dose that you were given may be enough to get your system over whatever reaction was causing the rash.

Here is information about how dexamethasone is processed (from this webpage - https://www.glowm.com/resources/glowm/cd/pages/drugs/d017.html) Please note that "PO" means by mouth (oral pills):

Pharmacokinetics
Inactive metabolites and small amounts of unmetabolized drug are excreted by the kidneys. Insignificant quantities are also excreted in feces; biologic half-life is 36 to 54 hours.

Route     Onset      Peak      Duration
P.O.      1-2 hr      1-2 hr      2 1/2 days
I.V.      1 hr      1 hr      Variable
I.M.               
acetate     1 hr      8 hr      Unknown
phosphate1 hr      1 hr      6 days

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Brief Answer:
I hope you are doing better.

Detailed Answer:
I'm sorry I did not see your return message sooner! I hope that your dermatology appointment provided some answers about the cause of the rash.

I don't think it is typical for a rash to come out all at once from bed bugs. They bite here and there over time. Still, if you are concerned, check the seams of your upholstery including your bed and any upholstered furniture for visible bed bugs. You can google bed bug images to see what they look like/size.

If the dermatologist hasn't already given you instructions, and if you see the rash coming back, then yes, you might need a total of around a week of a corticosteroid, usually starting with a higher dose and then tapering (but a taper is not always needed if the rx is just for a week).

Is the Benadryl helping? If you find it overly sedating you can switch to Zyrtec which is also a good antihistamine for itching, and can be taken in higher doses than what is recommended on the bottle. Usual dose is 1 tablet every 24 hours as needed, but for severe itching, can be increased up to 4 tablets/dy safely. I wouldn't increase to 4 right away, as 1 pill twice a day may be adequate.

I hope this helps and you are soon feeling better!

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