Never Share Your Earbuds With Someone Else. Here's Why
Those of us of a certain age remember sitting on the school bus next to a friend, heads together, each party with one earbud in their ear to share a single listening device. Setting aside the impracticality of this habit for things recorded in stereo instead of mono, this was an unsanitary practice that you should not engage in anymore.
If you're the type who's easily grossed out, that alone should make you rethink this particular form of generosity. Earwax, a natural discharge from the ear made up of skin cells, oil, tiny hairs, and the everyday dirt and dust you pick up all over your body (via MedlinePlus). Because it's slick and oily, it actually protects the ear by keeping bacteria from entering it. Most people don't have to worry about it, as it naturally works itself out through regular jaw-moving like eating and talking, while other people visit doctors to help clean their ears out, which is typically a pretty simple and safe procedure.
That said, it just looks gross! You've probably noticed how bits of wax stick to your earbuds when you take them out of your own ears — and it's especially squicky to see wax that comes from someone else.
It can spread infection
The more you use your earbuds, the more likely they are to irritate your ear canal over time, which can cause small cuts. Those cuts in turn have the potential to get infected, in part because when you're not using your earbuds, they're probably lying around in your gym bag, on your kitchen counter, or lost somewhere in your bedsheets, where they can pick up germs (via Yahoo). Introduce those new germs to those tiny cuts and the results are not pretty. Now imagine that happening to your friend, sharing your earbuds with them, and then popping them back in your own ears.
We have benign or beneficial bacteria living in and outside of our bodies all the time, ears included. But just as your gut flora is unique to you, so is the stuff living in your ears. Every body has its own particular needs, idiosyncrasies, intolerances, and strengths. Organ transplants may be cool and lifesaving, but there's nothing useful about a cerumen transplant. Keep your wax to yourself.
If sharing is inevitable, like if your brother swipes your earbuds without asking or your roommate gets yours mixed up with hers, at the very least, keep isopropyl (rubbing) alcohol around, either in pads or in a bottle that you can dip a Q-tip in. Before you use your buds in your own ears, sanitize them (via CNet). For any big crusties, carefully use a toothpick. Finish off with a swipe of alcohol pads. And don't put your earbuds inside your ears until they're dry!