On March 11, 2012, Bales was on his fourth combat tour stationed in Panjwai District of Kandahar Provence, Afghanistan when he left his post and killed 16 Afghans, including women and children, in two nearby villages.
During those deployments, Bales was taking mefloquine, but it was never recorded in his medical records.
Mefloquine, developed by the Army in the 1970s, has long been linked to disturbing neurological and psychiatric side effects.
In July 2013, the U.S. Food and Drug Administration (FDA) even revised its warning label for the drug, saying rare but sometimes permanent side effects include "dizziness, loss of balance, and ringing in the ears," as well as "feeling anxious, mistrustful, depressed, or having hallucinations."
Still, the FDA approved mefloquine for the prevention and treatment of malaria.
Shortly after that update, the Department of Defense said its policy for the drug was consistent with the FDA. Several months earlier in April, Dr. Jonathan Woodson, assistant secretary of defense for health affairs, ordered mefloquine to be used as a last resort if four other malaria drugs didn't work.
Questions swirl around mefloquine
The drug was most famously investigated after four soldiers from Fort Bragg killed their wives in 2002.
Two of the men later committed suicide before standing trial. But a military panel found the drug was an "unlikely" factor in the murders.
A CNN and United Press International (UPI) investigation in 2004 looked into the suicides of three Special Forces soldiers who had all taken the drug.
"None appears to have had acute marital problems, combat stress or other personal issues that would help explain their sudden plunge into violence," UPI said in a report.