Occasional phenibut use isn't going to be an issue. However, if you lack discipline it can be rather difficult to say no the next day and it can easily lead to an everyday thing. Dosage with Phenibut is everything and if you take too much at once or too little - the effects can either be way to overwhelming or under-delivering. Its best taken in 2 small doses multiple hours apart. Where you get it from is also an issue. Many of the leading nootropics dealers sell Phenibut that is highly questionable.
If you have severe performance anxiety your going to need something strong. As much as the fluffy recommendations to take chamomile tea and simply " take a few deep breaths " may sound - it simply does not harbor the strong effect that is needed. '
Although the supplement GABA gets a bad wrap, I know multiple people that say it has worked wonders for their anxiety issues. There are a ton of people that say that because it reportedly does not cross the blood brain barrier that it does not work.
My unpopular opinion is to consider looking into the Racetam family. Things like Aniracetam, Fasoracetam, Phenylpiracetam, Oxiracetam etc - or Even Adrafinil for occasional use. Whenever I have a big event, I take Aniracetam and it really works wonders.