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Some medication (certain antibiotics, thyroxine, PPIs) need to be taken on an "empty stomach". For most patients, that just means they take them after sleeping. However, certain circumstances can lead to that not being feasible.
How long after the last meal should these drugs generally be taken to be sure?
In my experience most anesthesiologists use 5 hours as the minimum time necessary for the stomach to be sufficiently empty to make it safe to administer anesthesia. However, their criteria is quite stringent because the consequences of the stomach not actually being empty can be severe (aspiration of vomit).
The National Health Service (UK) offers this guideline:
As a general rule, medicines that are supposed to be taken on an empty stomach should be taken about an hour before a meal, or two hours after a meal.
I've seen that guideline used in medication instructions so I believe it is widely accepted.
