This will be old news to you, but the menthol drops are STRONG. You'll want to mix it before using, putting it straight into the atomizer can make it a dedicated menthol atomizer.
This is going to vary with each user but here is what I've found with our menthol.
1 drop in a 12ml = slightly mentholated, much like a menthol analog.
2 drops in a 12ml = strong menthol flavor (upper limit of what you'll get with an analog)
3 drops in a 12ml = You now own a completely menthol flavored liquid.
Vaping it straight is generally a bad idea but safe. It's like eating two bananas and drinking a liter of Sprite. You can achieve the goal but won't like the result.
