E-liquids come in different strengths depending on how much nicotine they contain.

Zero strength nicotine level – 0mg
Low strength nicotine level – 6-11mg
Medium Strength nicotine level – 12-17mg
High strength nicotine level – 18-36mg

When choosing consider your current smoking habits and how much nicotine you are used to. Choose the lowest level to suit your needs as more nicotine than you are used to will leave you feel dizzy, lightheaded and generally unwell, the same as it would with traditional cigarettes if you swap between lights and a stronger brand containing more nicotine etc.