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According to my doctor, 4 hours. You can switch them up. Take acetaminophen, (Tylenol), then wait 4 hours and take Ibuprofen, wait another 4 hours and take acetaminophen and so on. :) 15 people found this useful.
If there's no underlying disease (liver, stomach, etc.), allergy, or illness... it is AS SAFE to take tylenol or ibuprofen once or twice a day at the max dose on the bottle, as it is to drink water, or eat carrots. Our bodies deal with them quite nicely. Not addicitve, filtered easily.