Nicotine is a potent parasympathomimetic alkaloid found in the nightshade family of plants and is a stimulant drug.
It is commonly used as a stimulant and found most commonly in the leaves of the tobacco plant.
Nicotine is a toxic chemical and proper safety handling when using nicotine liquids is very important. It can easily be absorbed through the skin so the use of gloves to protect hands is important as well as eye-protection and even clothing to prevent exposure to the skin is highly advised when mixing.
Most D.I.Y. mixers will usually purchase nicotine liquid at a high strength of 100mg/ml and reduce that when mixing. Vaping a liquid with more than 40mg/ml is dangerous. 30mg/ml is still risky but some users use this strength. The most common strengths tend to be around 15-20mg/ml for users that are using "Mouth hits" and a strength that's around 3-8mg/ml for "Lung hits." This is mainly because "mouth hits" tend to result in less vapour being delivered to the lungs because the user is pulling the vapour into their mouth only and then inhale (this is similar to how a regular analog cigarette is smoked.) With "Lung hits" the user is usually inhaling directly into the lungs so they are getting a significantly larger amount of vapour being delivered into the lungs each "hit."
Nicotine is a toxic compound and proper safety handling is important when you get into mixing e-liquids.
The use of gloves, protective eye-wear and even protective clothing is strongly advised.