Attorney General Martha Coakley's office said Sonja Farak, 35, of Northampton is charged with two counts of tampering with evidence, one count of possession of a Class A substance and one count of possession of a Class B substance.
Farak is a chemist at the Amherst Drug Laboratory, which the state said is used to store and analyze alleged controlled substances that have been seized by local and state authorities.
Members of the lab contacted the state police this past Friday when they noticed a discrepancy in their inventory.
They allege that, in one instance, Farak removed cocaine from a case, replacing it with a substance that no longer tested positive for the drug.
Farak will be arraigned Tuesday morning in Eastern Hampshire District Court.
Coakley's office says the investigation is ongoing.
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