Phenibut is a GABA agonist. You can try two things, neither of which I am going to necessarily recommend but which should work. Firstly, you could consume a competing GABA agonist with selective affinity for the receptor but a shorter half-life. The only one I can think of is THC which is the active component in marijuana. Secondly, you could try a GABA antagonist, which should increase the number of GABA receptors, making you more susceptible to GABA agonists. All benzodiazopens are GABA antagonists, such as xanax or klonopin.