This happened to me the other night. A patient had dangerously high blood pressure, and legitimately had reasons she could not take most of your first line blood pressure drugs.
The blood pressure needed to come down, and fast, so I decided to give nitroglycerin under the tongue. Unfortunately, the particular department only stocked a type that I was unfamiliar with, and didn't know the dose.
Here's what you do: ask the nurse to show you the available pills. If you want to look like you know what you are doing, pretend like you want to see because you are weighing your options, or like you doubt that this department will really have what you want.
In this case, there were 3 different dose pills of the drug. The highest dose pill was 6 times that of the low dose one, hinting at a wide therapeutic window.
Then, use one pill of the very lowest dose there is. Unless it is a strange drug, a drug that requires a very slow titration to full dose, or a drug like digoxin, you will rarely go wrong. Since I saw that my lowest dose was 1/6 of the highest one, I figured that even if the one pill didn't work, I could easily add another. She was naive to this drug.
Sure enough, a few minutes after sublingual administration, her blood pressure was reasonable - not great, but she wasn't going to have a massive stroke because I didn't know what to do.
Remember, as scary as it is to mess with a drug you don't quite know about, if it's common, the lowest dose pill is probably taken by millions of people every day.