British and New Zealand researchers behind the Cochrane Library review say the results are encouraging but that more studies are needed.
Electronic cigarettes work by vaporising a solution that usually contains nicotine.
The review drew on data from 662 smokers across two randomised trials, and found that about 9% of those who used electronic cigarettes were able to quit smoking by the one-year mark — more than twice the rate of those using nicotine-free placebo devices.
Among people who did not quit, 36% of those smoking e-cigarettes cut down on the number of cigarettes they were smoking by half, while 28% of those using the placebo devices were able to halve their cigarette consumption.
Only one of the two trials compared nicotine patches with e-cigarettes, and found their efficacy was similar.
One of the review’s authors, clinical psychology professor Peter Hajek, said that his team was limited by the small number of trials already available and the sample sizes in each.
Prof Hajek said authorities appeared wary of e-cigarettes and that this could have slowed down research efforts.
“[For example] we are running a trial where we wanted to have three sites: one in Spain, one in the Czech Republic and one in Queen Mary (University of London), and the Spanish ethics committee put a stop to it, saying you can’t give electronic cigarettes to smokers, without giving any explanation.”
This kind of institutional “uncertainty” could help explain why research in this area had not taken off earlier, he said.
Based on what little is known about e-cigarettes, Prof Hajek said he would support their wider use as a tool for smokers trying to quit.
“I think they should be used, but not as the first line of treatment. I think the smoker who is looking for help stopping smoking should be given treatments which have been proven clearly effective,” he said.
He stressed that although the review did not suggest any serious adverse effects occurred in e-cigarette users over the short to medium term, this did not mean they were completely safe.
“There may be an element of risk in using them. But we are not comparing them with nothing, or with fresh air; we are comparing them with cigarette smoke which kills — ultimately— one in two users, and the difference in health risks would be massive,” said Prof Hajek.
Deborah Arnott, the chief executive of health charity Action on Smoking and Health, said the study should stimulate more research “which is just what is needed in this area”.