Germany's health minister warned the public against untested medicines on Saturday, after the deaths of three people who had received a form of alternative cancer treatment.
"The administration of substances that are not approved as medicinal products and are in the stages of fundamental research through experiments is unacceptable," Health Minister Hermann Groehe told the Rheinische Post newspaper.
Groehe stressed that such practices could not be justified, even if the patient explicitly requested the treatment.
Three people died in late July after receiving an alternative form of cancer treatment at a clinic in western Germany.
The causes of death have not yet been determined and police have launched an investigation into the matter.
Investigators are working to establish whether the anti-cancer agent 3-Bromopyruvat played a role in the patients' deaths, a police spokesman said.
The substance is listed on the website of clinic in Brueggen am Niederrhein as a cancer-fighting agent. It is unclear whether the patients who died had taken the drug.
The complimentary healthcare practitioner who runs the practice has said he is cooperating with the investigation in a statement.
Officials in the district of Viersen had initially questioned the doctor's qualifications.