Hello! Can you please tell me which of the two expressions is better and how I can express the idea that the effect lasted throughout the whole period of working hours? Thank you in advance

In meanwhile, I have found out that the drug worked for/took effect for seven to eight hours, therefore during the whole? working hours.
How's this:

...the drug was effective for seven to eight hours, lasting throughout the work day.
In meanwhile should be either In the meantime, or Meanwhile.
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Thank you both!
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