During the first few hours after taking phenibut, you may experience dizziness or drowsiness. Do not drive or operate heavy machinery until you know how this drug affects you.  Taking phenibut may cause a number of side effects including drowsiness and dizziness. These side effects usually go away within a few days to a few weeks. If they become more severe or do not go away, talk to your doctor.  More serious side effects include difficulty breathing and even unconsciousness.  You may also experience the following more common side effects:  • Upset stomach (i.e., nausea) • Hangover effect