Eynulla Fatullayev's blood was tested over the weekend after authorities said they found 0.23 grams of heroin in his jacket after searching his prison cell in Baku on December 29.
Fatullayev's lawyer, Isakhan Ashurov, said the results do not mean the drug-possession charges against his client will be dropped.
Fatullayev said the drugs were planted on him.
His father, Emin Fatullayev, claims his son was framed by prison officials and says it would be impossible for drugs to be brought into his son's cell without the connivance of prison personnel.
If found guilty of the drug charges, Fatullayev could face three years in jail on top of his 8 1/2-year term for "for instigation of terror and tax evasion," ordered after a 2007 conviction.
Many international groups have accused authorities of manufacturing those charges for political reasons.
The European Court of Human Rights is finalizing Fatullayev's case challenging the Azerbaijani government's charges against him.