Phenazepam is a powerful benzodiazepine developed in the Soviet Union in 1975. It belongs to the same drug class as diazepam (Valium) and temazepam, and has been prescribed in Russia and some former Soviet countries since 1978 to treat anxiety, epilepsy, alcohol withdrawal, and sleep disorders. It is not licensed or approved for medical use in the United States, the United Kingdom, or most of Europe, which is why many people encounter it only as an unregulated substance sold online or on the street.
How Phenazepam Works in the Brain
Like all benzodiazepines, phenazepam targets a receptor in the brain that responds to GABA, the nervous system’s main calming chemical. Normally, GABA binds to its receptor and slows down nerve activity. Phenazepam doesn’t attach to the same spot on the receptor that GABA does. Instead, it binds to a nearby site and nudges the receptor into a state where it responds more strongly to whatever GABA is already present. The result is amplified sedation, muscle relaxation, and reduced anxiety.
Research in the British Journal of Pharmacology describes this process as a “modest de-stabilization of the closed state” of the receptor, meaning phenazepam essentially makes it easier for the receptor’s channel to open and let calming signals through. This is the same basic mechanism behind every benzodiazepine on the market, but phenazepam stands out for its potency and its unusually long duration of action.
Why It Lasts So Long
Phenazepam has one of the longest half-lives of any benzodiazepine. When taken orally, its elimination half-life ranges from roughly 49 to 301 hours, with a median around 103 hours. That means after a single dose, it can take over four days for just half of the drug to leave your body. For comparison, diazepam’s half-life is typically 20 to 100 hours, and many commonly prescribed benzodiazepines clear in under 24.
This extreme duration creates a specific danger: people who don’t feel the expected effect quickly may take a second or third dose, not realizing the first one hasn’t peaked yet. The drug accumulates, and what seemed like a moderate amount becomes a serious overdose hours later. Injected phenazepam clears faster, with a half-life of about 15 hours, but oral use is far more common outside clinical settings.
Medical Uses in Russia and CIS Countries
In the countries where it is prescribed, phenazepam is used for a range of conditions. These include generalized anxiety, epilepsy and seizure disorders, alcohol withdrawal syndrome, and insomnia. Typical prescribed doses start at 0.5 mg taken two to three times daily, with a maximum of 10 mg per day. For sleep problems, a single dose of up to 4 mg before bed has been documented in clinical references.
These dosages highlight how potent the drug is. A 0.5 mg tablet of phenazepam produces effects roughly comparable to 10 mg of diazepam. People who obtain it without medical guidance often have no way to gauge an appropriate dose, especially when it arrives as a powder or in tablets of unknown strength.
Side Effects and Risks
Phenazepam produces the full range of benzodiazepine side effects, but its potency and long half-life make those effects more dangerous and less predictable. Common effects include heavy sedation and drowsiness, impaired coordination and balance, slurred speech, confusion and memory blackouts, and muscle weakness.
At higher doses or when combined with alcohol, opioids, or other sedatives, phenazepam can suppress breathing to a life-threatening degree. Because the drug takes so long to clear, the window of danger stays open far longer than with shorter-acting benzodiazepines. Someone who appears fine several hours after taking it may deteriorate as the drug continues to be absorbed and redistributed.
Repeated use leads to physical dependence, just as with other benzodiazepines. Withdrawal can produce rebound anxiety, insomnia, tremors, and in severe cases, seizures. The long half-life means withdrawal symptoms may not begin until days after the last dose, which can catch people off guard.
Legal Status Outside Russia
Phenazepam has never been approved by the FDA or licensed in the UK or EU for medical use. The U.S. Drug Enforcement Administration has flagged it as a substance of concern, listing street names like “Bonsai,” “Soviet Benzo,” and “Fenaz.” Several countries have moved to control it after a wave of hospitalizations and deaths linked to its sale as a “legal high” or “research chemical” in the late 2000s and early 2010s. The UK classified it as a controlled substance in 2012, and Sweden, Finland, and several other countries followed.
Despite these restrictions, phenazepam continues to circulate online, sometimes sold as a different benzodiazepine entirely. It also appears in counterfeit tablets marketed as other prescription drugs.
Detection on Drug Tests
Standard urine drug screens for benzodiazepines use a method called immunoassay, which looks for structural features common to the drug class. Because phenazepam closely resembles prescription benzodiazepines, it may trigger a positive result on these routine tests, but this is not guaranteed. Some designer and unusual benzodiazepines slip through immunoassay screening entirely.
Confirmation testing with more advanced techniques like liquid chromatography can specifically identify phenazepam and its main metabolite, 3-hydroxyphenazepam, in urine samples. Given the drug’s extremely long half-life, it may remain detectable for weeks after a single use, though detection windows vary depending on the dose, individual metabolism, and the sensitivity of the test being used.