While many say the long-term effects are unknown, a recent review of 81 studies of e-cigarettes found that - compared to smoking - they are "likely to be much less, if at all, harmful to users or bystanders".
The review, conducted by researchers at Queen Mary University of London, also said the devices would benefit smokers unwilling or unable to quit. Cigarettes contain more than 4000 chemical compounds, many of which are toxic or carcinogenic, including tar, carbon monoxide, nitrogen oxide and ammonia.
However, the liquid used in e-cigarettes usually contains just nicotine dissolved in a solution of water and propylene glycol - a substance which has been approved for use in food.
Some health professionals claim swapping regular cigarettes for e-cigarettes is almost as good as stopping smoking altogether, while others claim nicotine addiction is still a serious concern.
The British Heart Foundation states that nicotine can raise blood pressure, making your heart work harder, by stimulating adrenaline.