The Norwegian who admitted to the Oslo bombing and Utoya island massacre that killed 77 people had traces of drugs in his blood at the time of his arrest.
Samples were taken from Anders Behring Breivik after his arrest on July 22, his lawyer said on Sunday.
"The results are not complete but he (Breivik) has said he took a cocktail of medicines," Geir Lippestad told the VG newspaper.
Prosecutor Christian Hatlo on Saturday told broadcaster NRK that blood tests taken after Breivik's arrest on July 22 indicated his client had taken some form of drugs. He did not offer details.
Hatlo also said police were becoming increasingly convinced Breivik carried out the attacks alone, as the 32-year-old has stated.
Breivik was remanded in custody on July 25. He has admitted to planting a car bomb in Oslo that killed eight and a shooting rampage at a Labour Party youth camp that left 69 dead.
Police have interviewed Breivik for about 30 hours, and a main line of questioning is to determine if there were accomplices.
"As the days pass we are more and more certain he managed to carry out the attacks alone," Hatlo told Sunday's edition of the Aftenposten daily, sating Breivik's explanations seemed to tally.
Police have questioned Breivik about how he made the bomb and about the vehicles he used in the car bomb and to drive to Utoya.
Hatlo said initial demands by Breivik - such as calling for the government to resign - were no longer an issue in the interviews. His demands were now more in line with usual requests such as access to books or TV series while in custody, the prosecutor told Aftenposten.