April 5, 2020 / Victoria Forster, Contributor
Researchers have warned that hydroxychloroquine (HCQ) and chloroquine (CQ), two similar drugs repeatedly touted by President Trump to be promising treatments for COVID-19, may be toxic when combined with a common diabetes drug.
The new study was published online on scientific pre-print server BioRxiv and shows that 30-40% of mice treated with a combination of HCQ or CQ and diabetes drug metformin, died. Treatment with the same dose of either drug alone had no effect on the survival of the mice. 
HCQ and CQ are typically used to treat malaria and autoimmune diseases such as rheumatoid arthritis and lupus, but have also shown some early promise in the treatment of certain types of cancer, with several clinical trials ongoing.
“Our interest in this combination arose because both drugs individually have been shown to have anti-tumor effects in pancreatic cancer,” read a statement from two of the authors of the paper: Chi Dang, director of the Ludwig Institute for Cancer Research and Anirban Maitra, scientific director of the Center for Pancreatic Cancer Research at MD Anderson Cancer Center. “To our utter surprise, both HCQ and CQ when combined with metformin resulted in a surprising death rate in 30-40% of mice. In contrast, there were no deaths in the single treatment groups,” said the authors.