Reheating your already brewed coffee will only help to evaporate a good portion of whatever was left of the coffee’s aroma. This means that even if the caffeine content is the same as 4 hours ago, it will seem lower because the taste is mostly gone. Sure, it will taste a bit like coffee, but it will be off.
Why does reheating coffee make it bitter?
Does coffee lose caffeine over time?
No, coffee keeps its caffeine levels in time. Unless you specifically remove the caffeine from the beans to make it a decaf version.
Does caffeine evaporate in coffee?
No, caffeine does not evaporate. Unless you specifically remove the caffeine from the beans to make it a decaf version, your coffee will retain its caffeine. If you brew a cup of coffee now, and set it on the counter for 8 hours and come back to drink it, the caffeine will still be there.