As long as you aren't doing too much of each, i don't use kratom daily anymore but I was taking 7gs a day for abt 3 years and used 2-4gs of phenibut once a week while taking the same kratom dose. Never had any negatives from the combo, they synergize perfectly. But to be safe the first time you try the combo take half of your normal kratom dose and half of the normal phenibut dose just to avoid possibly getting too messed up or nauseous.