Perfect rhymes (but not common words):
* drugg (a variant spelling of "drug")
* mugged (past tense of "mug")
* slugged (past tense of "slug")
* strugg (an archaic word meaning "to struggle")
Near rhymes (words that sound similar, but don't perfectly rhyme):
* frug (a type of dance)
* fug (a slang term for something unpleasant)
* bug (an insect)
* rug (a floor covering)
Ultimately, the best rhyme for "frugg" will depend on the context and the desired effect.