Medical Definition of THC
: either of two physiologically active isomers C21H30O2 that occur naturally in hemp plant resin or are synthetically prepared:a: one that is the chief intoxicant in marijuana and is used medicinally—called also delta-9-tetrahydrocannabinol, delta-9-THC; see dronabinolb: one that is present in marijuana only in minute quantities
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