Quitting tobacco may not be easy as taking up electronic cigarettes.

A study published in today's New Zealand Medical Journal shows the products contain more nicotine than their labels say and are stronger than they used to be.

It's found the products are still far less toxic than regular cigarettes and far safer to use.

But Otago University marketing expert Janet Hoek says the report doesn't show e-cigarettes are harmless and many smokers use both regular and electronic products.