This neologism, I hope, conveys the meaning of the source term.

Washout period
The stage in a cross-over trial when treatment is withdrawn before the second treatment is given. Washout periods are usually necessary because of the possibility that the intervention administered first can affect the outcome variable for some time after treatment ceases. A run-in period before a trial starts is sometimes called a washout period if treatments that participants were using before entering the trial are discontinued.

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Note added at 1 day6 hrs (2008-04-19 01:33:20 GMT)
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वस्तुतः होता यह है कि एक परीक्षण उपचार के बाद जब रोगी पर दूसरा परीक्षण किया जाना होता है और उसे नई दवाई दी जानी होती है तो एक निर्धारित अवधि तक प्रतीक्षा की जाती है ताकि पिछली दवाई का असर समाप्त हो जाए और नई दवाई दी जा सके। यही अवधि Washout period कहलाती है।

Prior to study participation, patients who are pretreated with XXX plus another YYY medication will stop their current therapy (except for the allowed XXX therapy) and enter a medication wash-out period.