The Greens have raised allegations in Federal Parliament about mistreatment of former Guantanamo Bay inmate David Hicks.

Greens senator Penny Wright says there is new evidence Mr Hicks was drugged against his will while he was held in custody.

The Senate has voted down a motion for a fresh independent inquiry into the Australian Government's role in the case.

Senator Wright says an inquiry is needed to investigate recent reports Mr Hicks was forced to take an anti-malaria medication that can cause side effects like psychosis and brain damage.

"How long has the Government known about this? Who knew about it?" she asked.

"And what steps is the Government taking to obtain further information about the mistreatment of an Australian citizen?"

Labor frontbencher Joe Ludwig has told the Senate that United States authorities have conducted two investigations that found no evidence of mistreatment or abuse.

"I think we should put it in perspective that this government provided Mr Hicks with considerable assistance during that period," he said.