Before he launched his July 22nd bomb attack on a government building in Oslo and went on a shooting rampage on Utøya island, Breivik said he had taken an an ECA stack, a combination of ephedrine, caffeine and aspirin.
This mix is illegal in Norway and the United States among other countries but is popular among body-builders and is often used for weight loss and as a stimulant.
"He was slightly to moderately under the influence of a high affecting the central nervous system," Professor Jørg Mørland of the Norwegian Institute of Public Health told the Oslo district court on the 28th day of Breivik's trial.
The 33-year-old right-wing extremist has said he took 50 percent more than the normal ECA stack dose authorized in the European Union only a few hours before his attacks.
Breivik has told the court the drug cocktail allows "the brain to absorb more oxygen and this results in better physical and mental performance."
According to Mørland, ephedrine boosts both self-confidence and the willingness to take risks.
The effect of the substances was nonetheless limited, the expert said, saying that the amount Breivik had taken was equivalent to "four to six big cups of moderately strong coffee."
In the months leading up to the attacks, the 33-year-old extremist also took anabolic steroids to help build muscle mass.
"In my eyes, there was no additional effect due to the steroids, but I cannot rule out that they could have contributed to his aggressiveness and agitation," Mørland told the court.
Breivik has confessed to the twin attacks but has refused to plead guilty, insisting they were "cruel but necessary" to stop the ruling Labour Party's "multicultural experiment" and the "Muslim invasion" of Norway and Europe.
While he has been charged with committing acts of terror, the focal point of his ongoing trial is to determine the question of his sanity.
The ruling of the five judges on that issue, when they hand down their verdict in July, will determine whether he will be sent to prison or a closed psychiatric ward.