First of all, if you’re just talking about whether or not coffee goes bad over time, I’ll save you some time:
Yes. It definitely, 100% does go bad.
Now that that’s out of the way—Does old coffee have less caffeine?
Compared to the other elements in coffee, caffeine is a fairly stable chemical. It tends to last for months without losing any significant amount of potency.
That means if you have old coffee, the caffeine in it is still going to be just about as potent as it was when you originally bought it.
In fact, caffeine is one of the substances in coffee that changes the least over time.
For that really old coffee your mother-in-law gifted you two years ago, there’s a very good chance the caffeine is the substance that’s closest to its original potency.
Obviously, caffeine potency is independent of things like, say… smell, taste, rancidity, and safety.
So if you can stomach it—literally—old coffee isn’t significantly less potent caffeine-wise than new, good tasting coffee.
(This is a good time to point out that just because you can stomach something doesn’t mean you should.)
Believe it or not, even coffee grounds that have been used still retain a lot of caffeine. Reused grounds make absolutely horrible coffee, but they do contain a lot of caffeine.
So even in the sense of “Does old (used) coffee get less potent?” the answer is “Yes, but not as much as you might think.”
Again, even if you’re keeping your coffee in proper, optimal storage conditions, caffeine potency will probably be the “last man standing” as far as the attributes of that coffee go. Time won’t do the oils, flavors, or aroma of coffee any favors, but the caffeine will still be there even several years after it was roasted.
But just throw it away, dude. Don’t do this.
The amount of time you’d ever want to drink coffee that’s far past its roasting date is a pretty short window—nil if you’re a true coffee snob.
If you’re considering brewing some coffee that’s many months, or several years old, you really have no right to call yourself a snob of any sort.
But there’s a middle ground… you can be smart and good to yourself with the coffee you drink without crossing into snob territory.
But if you ever find yourself in a Target aisle talking loudly to strangers about how “mixing beans from different regions is an abomination!”—it might be time to check yourself.
So now that you know caffeine doesn’t really expire, but the coffee containing it does, what can you do with this knowledge?
Well… I guess if you ever have to emerge from an underground shelter to brave a post-apocalyptic wasteland, and need your caffeine fix, you’re in luck. Sorta.