The Australian was affected by a combination of anti-depressants and the painkillers he was taking for a shoulder injury, according to his manager, James Erskine.
Police spoke to Thorpe after residents living near his parents' home in Panania, Sydney, reported a man allegedly breaking into a van yesterday.
He was taken to Sydney's Bankstown Hospital for assessment and later transferred to a rehab facility.
Mr Erskine said: "He is in rehab for depression."
He said Thorpe thought he was sitting in his friend's car.
Mr Erskine said Thorpe, 31, had been taking anti-depressants and medication for his shoulder but had not been drinking alcohol.
Thorpe, who lives in Switzerland, had been staying with his parents.