A new study raises doubts about the usefulness of e-cigarettes in helping cancer patients quit smoking.

The study was published in Cancer, the journal of the American Cancer Society. The researchers studied 1,074 cancer patients who smoked and were enrolled in a tobacco treatment program at a cancer center. There was a three-fold use in e-cigarettes among participants from 2012 to 2013—from 10.6 percent to 38.5 percent. At the beginning of the study, e-cigarette users were more likely to be nicotine-dependent than those who didn’t use e-cigarettes. They had tried to quit smoking more often, and were more likely to be diagnosed with cancer of the lung or head and neck. By the end of the study, e-cigarette users were as likely as non-users to be smoking.

There is a lack of long-term scientific evidence of the safety of e-cigarettes, Reuters reports. While some experts think they could lead to nicotine addiction and become a gateway to smoking, others say e-cigarettes have the potential to help millions of smokers quit.

A study published this summer concluded that allowing e-cigarettes to compete with regular cigarettes might reduce deaths and illness caused by tobacco. The researchers reviewed 81 previous studies on the use and safety of e-cigarettes. The researchers noted that although the long-term health effects of e-cigarette use are unknown, compared with conventional cigarettes they are likely to be much less harmful to users or bystanders.

A group of leading lung health organizations recently urged governments to ban or limit the use of e-cigarettes until more is known about the devices’ health effects. The American Medical Association recently called for reining in the sale and marketing practices of companies that make e-cigarettes.