Scientists are working with three compounds that show promise in treating pain, without the side effects of opioids, CNBC reports.

One experimental drug, oliceridine, is in the final stages of human trials. It is administered intravenously. Studies show the drug relieves pain like morphine, without causing respiratory depression or constipation. However, it is no less addictive than morphine and it can only be used in hospitals. The company that makes oliceridine, Trevana, is developing an oral form of the drug.

A second drug, derived from the kratom plant, has been studied in mice. Researchers found the drug provided pain relief, but the mice did not build up a tolerance to the drug, and did not show signs of addiction.

A third drug in early testing, PZM21, has also been tested in mice. One study showed the painkilling effects of the compound lasted much longer than morphine, without causing respiratory depression and constipation.