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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.
In other words, if you started with a weight-dependent dose of Tylenol or its generic, acetaminophen, wait four hours and then give Advil or the generic ibuprofen. If you started with Advil, give Tylenol four hours later. After another four hours, you can give a second dose of the first medication, and four hours later, you'll give a second dose of the second medication.