Last reviewed September 2026
Phenibut slows down the nervous system, so many of its side effects come from too much sedation. Because nobody checks the strength of products sold in the U.S., it’s hard to know how much of the drug you’re actually getting.
What poison centers see
In U.S. poison center cases from 2009 to 2019, the most common effects were:
- agitation (about 30 percent of cases)
- drowsiness or sluggishness (about 29 percent)
- fast heartbeat (about 22 percent)
- confusion (about 21 percent)
It may seem odd that a sedating drug causes agitation, but doctors describe this “paradoxical” agitation with phenibut alongside the drowsiness.
Signs of too much phenibut
German toxicologists reviewing the evidence in 2024 listed heavy sleepiness, confusion, hallucinations, agitation and, in severe cases, coma. Taken with alcohol or opioids, phenibut can also slow breathing.
Effects of regular use
People who use phenibut often tend to need more of it over time. Between doses or after stopping, they can end up with anxiety and sleep problems that are worse than what they started with. That’s withdrawal, and it can become serious. See phenibut withdrawal.
Driving and work
Phenibut can cause drowsiness and confusion that make driving or running machinery dangerous.
Call 911 if someone:
- can’t be woken up or keeps nodding off
- is breathing slowly or irregularly
- has a seizure
- is severely confused, hallucinating or extremely agitated
For advice on a possible overdose that isn’t an emergency, call Poison Help at 1-800-222-1222. It’s free and open 24 hours a day.
Sources
- CDC, Notes from the Field: Phenibut Exposures Reported to Poison Centers — United States, 2009–2019, MMWR, 2020
- Bonnet U. et al., Phenibut: an illegal food supplement with psychotropic effects and health risks, Deutsches Ärzteblatt International, 2024
- World Health Organization, Pre-Review Report: Phenibut, 2021