Phaclofen, or phosphonobaclofen, is a selective

antagonist An antagonist is a character in a story who is presented as the main enemy or rival of the protagonist and is often depicted as a villain.GABA_B receptor. It was the first selective GABA_B antagonist discovered, but its utility was limited by the fact that it does not cross the blood brain barrier.

References

GABAB receptor antagonists Phosphonic acids 4-Chlorophenyl compounds {{nervous-system-drug-stub