Caveat to my posts. I'm a lawyer, but not a tax lawyer, and this is not legal advice.
Anywho, TeddyBearCowboy is probably correct, but for the wrong reason, and there's a bit of ambiguity. Diapers most likely aren't a drug or medicine under the FSA definitions, they're a medical device. Publication 969 actually says to reference a different document, publication 502, for these definitions. If you check out
publication 502, it has a specific thing about diapers not being covered, sadly. Specifically, it says
Now, I should note that this is ambiguous. What counts as "a particular disease" and who gets to decide that is not clear. Almost certainly, if a doctor actually writes you a prescription for diapers, that counts as a particular disease and you'd get to be reimbursed for the use of your FSA to buy the diapers. But what else might count? Well, if you have a particular disease of any sort and you've got a doctor's diagnosis and diapers are helping deal with that, even if the doctor didn't explicitly prescribe you diapers, that might still count. After all, it doesn't say who gets to decide whether they're needed or not. Even more broadly, it doesn't say what proof is required for having a disease, so there's at least an argument that if you are suffering from a particular disease, even if a doctor hasn't confirmed that, and you need diapers to help you, you're still okay. That still leaves out some ABDLs, I think, since most of us would not classify our ABDL interest as a disease (and indeed, our community has taken pains in the past to ensure that people do not view us as diseased). So if you're buying strictly for pleasure, you probably are violating your FSA, but if you've got anything medical that the diapers are helping with, you're probably okay.
Although AGAIN, this is not legal advice, it's a very broad reading of this published IRS guidance by a person who does not specialize in this area of law. Do not assume you're fine just because you fit into a category I thought was probably fine or vice versa. If you get audited and need help, look around locally for a lawyer or accountant that can assist you.