Professor Des Cox:

Varying percentages are quoted across different studies. For instance, with regard to adolescents, the 2018 Planet Youth survey, which looked at 4,000 adolescents, stated that 22% used nicotine products, and of that 22%, 5% were e-cigarette users only, 7% were conventional cigarette smokers and the highest percentage was dual use at 9.3%, so dual use is common.

Other randomised control trials show that dual use can be up to 60% of users. It can be very high.