I am sorry, what I said was a bit confusing. I meant that having to go completely under anesthesia and simultaneously being on meth could very well be deadly. The anesthesiologist has to calculate exactly how much medicine to give you based on many factors, and meth will completely throw that off, requiring much more than is safe, and they may very well figure out that you're on drugs.
What is the procedure if you don't mind my asking?
The reason they're giving you a sedative in addition to local anesthesia is to make you relax. If you're tweaked out, you definitely won't be relaxed and the procedure may be more painful and stressful for you than it would have otherwise been.
At least 16 hours would be wise. Really good meth can last up to 18 hours. Maybe longer, but I've never heard of that from anyone.