“The question is: 'When do these therapies start to overstep the benefit?' ” says Andrew Ziskind, M.D., a senior vice president at Premier Academic Alliance.. “That’s where you get the tension between the traditional health care system and the alternative therapies.”

Many complementary therapies have little scientific evidence to back them up. Few large, high-quality studies have been done to test their efficacy. For example, several studies have looked at Reiki’s impact, but they are small and often lack a control group. Results are difficult to compare because the trials study different endpoints, such as anxiety, pain or depression, and typically enroll patients with a specific medical condition or who were undergoing a specific procedure.

Related: Consumers Drive Movement for Complementary and Alternative Medicine

The National Center for Complementary and Integrative Health concludes that although Reiki hasn’t been shown to have any harmful effects, it hasn’t been clearly shown to be useful for any health-related purpose.

A reason for the shortage of high-quality studies is that complementary practices can't rely on a research funding source equivalent to pharmaceutical and device maker research funding. But consumer demand has sparked an increase in academic and federal research.

Some physicians oppose the use of valued research dollars to study complementary practices. In an opinion piece in a 2014 issue of Trends in Molecular Medicine, two prominent physicians called for an end to clinical trials of these “highly implausible treatments.”

Health insurers don’t cover the majority of complementary therapies because of the lack of solid evidence supporting their effectiveness. Acupuncture increasingly is an exception because it has been studied more intensively than most other practices and has been found to ease some types of chronic pain.

The physician community’s views are mixed on complementary medicine. “There are always going to be some physicians who are more open and flexible, and then there are going to be others who are more resistant,” Ziskind says.