Tunisian swimmer Oussama Mellouli was banned for 18 months Tuesday for a doping violation and stripped of the gold and silver medals he won at the world championships.Mellouli, who tested positive for amphetamines at a meet in Indiana on Novemebr 30, 2006, was suspended by the Court of Arbitration for Sport after an appeal for a 2-year ban was filed by swimming’s world governing body.The ban, which dates back to the time of the doping offence, leaves Mellouli eligible to compete at next year’s Beijing Olympics.Mellouli won gold in the 800-meter freestyle and silver in the 400 freestyle in March at the world championships in Melbourne. He was the first Tunisian to win a gold medal at a world championships.CAS ruled that those medals and all of Mellouli’s results back to November 30, 2006, are annulled.CAS decided against the 2-year ban because Mellouli, who was studying at the University of Southern California at the time of the positive test, admitted he took the substance.“The swimmer admitted that he had taken an Adderall pill two days earlier to prevent him from falling asleep so that he could finish writing a report which counted towards his final university diploma,” CAS said.