1. something that is to happen or has happened to a particular person or thing; lot or fortune. 2. the predetermined, usu. inevitable, course of events. 3. the power or agency that determines the course of events.
Alternating between ibuprofen and acetaminophen every 2 hours because you have to wait 4 hours for each of them to take a second dose of the same thing. So instead you take a dose of Acetaminophen now, then Ibuprofen in 2 hours and back to Acetaminophen 2 hours later as needed.
There are two ways of approaching this question: one of how long is it effective in your system (providing the effects for which you took the drug) and the other of how long until it is eliminated. Effectiveness: The half-life of ibuprofen (the time after which half the dose has been eliminated) is about 2 hours.