You'd have to elaborate as there really isn't a concept of database record order that you can rely on, outside of a query that's ordered by a reliable field. What could provide a reliable sort order field would be one that is date/time but is formatted to include the time component. You'll need that to make an hours/minutes calculation. Also, what other field ties one time to another? Plus, do you really want to store this calculation or will you always want to compare the last dosage time stamp to Now? If the comparison is only really to Now, a form ought to be providing the calculation based on the last dosage timestamp for a patient. Without knowing the nature of the db's purpose, I can't say it's a good or bad idea. Maybe all you're after is the duration between successive events for one particular person. If you mix that duration with text, you won't easily be able to make comparisons or calculations based on the duration.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.