They legally have to take your long term into account.
The problem here is that the damage has already been done. The test is already failed. So, your options here are limited. Claiming "unintentional ingestion" is, on your facts, about the least successful option.
You are asking them to believe you, that you had no idea that you took the medication. You'd essentially have to get in front of the board and lie. It's harder than you think to lie convincingly.
That defense really should be limited to those with absolutely no choice, where the truth will get them into trouble because the truth isn't defensible at all.
Here, the truth is defensible. You simply had an outdated prescription which, as soon as you went to the doctor, was renewed.
A board is going to be able to understand that and, based on your long history, they'll want to believe that.....because it's true and you have plenty of evidence to back it up.
I think you weaken your position by trying to rely on the last resort defense of those with no excuse for their behavior at all.