The world sports tribunal said there were aggravating circumstances including Jeptoo's "deceptive and obstructive" behaviour, which warranted doubling the original ban against the 2014 Chicago and Boston Marathon winner.
The marathon titles were taken away from Jeptoo, now 35, who was also ordered to pay 15 000 Swiss francs ($15 000/13 850 euros) in legal costs to the world athletics body, the IAAF.
The IAAF had appealed to the CAS to get the ban extended against Jeptoo who was revealed to have failed a test for the blood-doping EPO drug on October 24, 2014, 12 days after winning the Chicago Marathon.
"The panel found to its comfortable satisfaction that the athlete used rEPO over a period of time to enhance performance," said a CAS statement.
It added that there was "undisputed" proof that the substance had been injected by a doctor.
"The athlete provided various differing accounts of the circumstances leading up to the injection and also regarding her relationship with that doctor."
The CAS judges said they were "comfortably satisfied that there are aggravating circumstances in the case at hand as it was obvious to the panel that the athlete used rEPO as part of a scheme or plan."