Paul Richards, aged 35, from Sutton Coldfield, and Baljit Singh Sunner, 36, from Small Heath, died within hours of each other on the oncology ward at Heartlands Hospital.
Richards was in remission and successfully battling back from the cancer when he died, the Birmingham Mail says.
Hospital bosses today confirmed a "mistake" was made over treatment on Ward 19 and an internal inquiry is under way into whether that error played a part in the deaths.
A hospital spokesman said: "The doctor and two nurses involved have not been suspended but are currently not working within the hospital and are deeply upset by the deaths.
"It has already been established that the two men received a higher dosage than normal.
"The coroner will look at whether the mistake made was directly responsible for the patients dying or a causational factor."
Both patients, who were battling different forms of cancer and leukaemia, received the higher than normal dose of a drug during the day of July 20.
Richards died that night and Sunner the following morning, at the hospital in Bordesley Green.
An official source revealed the patients were given five times the dose they should have received. The medication was not a cancer drug, but it eases side-effects of gruelling cancer treatment.
Sunner, who lived alone in a flat in Small Heath but worked at Michael's Drinks Stop, in Station Road, Stechford, died while his mother was away on holiday in India.
His nephew Jas Sunner said the family was devastated by the death.
Richards widow, Lisa, is battling to come to terms with the death with their young children and said: "Everyone is so shocked and angry and too upset to speak at the moment."
Dr Mark Goldman, chief executive of Heartland Hospital, said: "We are carrying out a detailed investigation into the clinical care given to both of these patients.
"This will be presented to both families and to the Birmingham coroner, and it will form part of the coroner's inquiries.
"We have already met both families and expressed our deepest sympathy."