Several categories of drugs can cause blackouts, meaning periods where you’re conscious and functioning but your brain fails to record new memories. Alcohol is the most widely recognized cause, but prescription sedatives, sleep medications, anti-anxiety drugs, and certain illicit substances all carry the same risk. The common thread is that these drugs interfere with the hippocampus, the brain region responsible for converting short-term experiences into lasting memories.
How Drug-Induced Blackouts Work
A blackout is not the same as passing out. During a blackout, you’re awake and may appear relatively normal to others, but your brain stops filing away what’s happening. This is called anterograde amnesia: the inability to form new memories while the drug is active in your system. The hippocampus essentially goes offline, so short-term experiences never get transferred into long-term storage. Because those memories were never created in the first place, no amount of prompting or reminders can bring them back.
Blackouts come in two forms. A fragmentary blackout (sometimes called a “brownout”) leaves you with patchy, incomplete memories that may partially return when someone gives you cues. A complete, or en bloc, blackout wipes out a solid block of time with no possibility of recall. As one participant in a study on memory impairment described it: “A brownout is hazy and you remember parts and bits. A blackout is a really solid chunk where there’s just absolutely no memory.”
Benzodiazepines
Benzodiazepines are among the most potent blackout-causing prescription drugs. These anti-anxiety and sedative medications, including diazepam (Valium), lorazepam (Ativan), alprazolam (Xanax), and midazolam (Versed), work by amplifying the activity of GABA, the brain’s main calming chemical. They bind to receptors on nerve cells and cause those cells to become less excitable, producing sedation and reducing anxiety. The same mechanism that calms the nervous system also suppresses the hippocampus’s ability to encode memories.
The amnestic effect of benzodiazepines was first documented in 1972, when intravenous diazepam impaired recognition memory in 90% of study subjects. The effect isn’t limited to high doses or injection. Oral diazepam impairs the ability to recall stories heard after taking the drug. Lorazepam taken by mouth produces verbal memory deficits that persist for hours. People with certain genetic variations (specifically carriers of the APOE4 gene variant associated with Alzheimer’s risk) show even more persistent long-term memory deficits after lorazepam compared to non-carriers.
Sleep Medications (Z-Drugs)
Prescription sleep aids known as Z-drugs, including zolpidem (Ambien), eszopiclone (Lunesta), and zaleplon (Sonata), are particularly notorious for causing blackouts paired with complex behaviors. People on these medications have driven cars, cooked meals, made phone calls, and left their homes, all while essentially unconscious and with no memory of any of it afterward.
The FDA added its strongest safety warning, a boxed warning, to these medications after identifying 66 cases of complex sleep behaviors resulting in serious injuries or death over a 26-year reporting period. Those injuries included falls causing skull fractures and broken hips, accidental overdoses, drowning, burns, carbon monoxide poisoning, and motor vehicle collisions where the patient was driving. Twenty of those 66 cases were fatal. Zolpidem accounted for the vast majority, with 61 of the 66 cases. Patients almost never remembered the events.
What makes Z-drug blackouts distinct is the combination of amnesia with apparently purposeful activity. Someone in an alcohol blackout is usually visibly intoxicated. Someone in a Z-drug episode may appear coordinated enough to perform complex tasks while having zero awareness of what they’re doing.
GHB and Rohypnol
GHB (gamma-hydroxybutyrate) and Rohypnol (flunitrazepam) are both associated with drug-facilitated sexual assault precisely because they cause rapid, deep amnesia. GHB produces a sudden loss of consciousness sometimes called “G-napping,” along with periods of anterograde amnesia. It has a half-life of just 27 minutes and becomes virtually undetectable in urine within 12 hours, making it extremely difficult to identify after the fact.
Rohypnol is a potent benzodiazepine that works through the same GABA mechanism as prescription benzodiazepines but is far more powerful. It produces profound sedation and dense amnesia. Standard toxicology tests can only detect it in blood and urine for up to 72 hours after ingestion. Both drugs are colorless and odorless in many forms, which is why they’ve earned the label “date rape drugs.” The rapid onset, deep amnesia, and fast clearance from the body create a situation where victims genuinely cannot recall what happened to them.
Alcohol
Alcohol remains the most common cause of drug-induced blackouts. It primarily interferes with the formation of new long-term memories while leaving previously stored memories and short-term recall relatively intact. This is why someone in a blackout can carry on conversations and navigate their environment in the moment but have no record of it the next day.
Not everyone who drinks heavily blacks out, and the threshold varies significantly between individuals. Genetics play a substantial role. Twin studies estimate that 43% of the variation in blackout liability comes from genetic factors, with heritability estimates as high as 53% in men and about 20% in women. People with naturally low sensitivity to alcohol’s effects are at higher risk, likely because they drink more before feeling intoxicated, reaching the blood alcohol levels that shut down memory formation without the warning signals that stop other people earlier.
Anticonvulsant Medications
Topiramate, a medication used for epilepsy and migraine prevention, causes cognitive side effects more frequently than other drugs in its class. An estimated 10 to 40% of people taking topiramate experience some form of cognitive impairment, including problems with verbal fluency, verbal learning, and both short-term and working memory. The severity scales with the drug’s concentration in the blood: for each unit increase in plasma concentration, working memory accuracy drops by roughly 3.6%.
Topiramate doesn’t typically cause the dramatic, total blackouts associated with benzodiazepines or alcohol. Instead, it creates a more persistent fog where word-finding becomes difficult, learning new information feels harder, and memory for recent events becomes unreliable. For some people, this can be disruptive enough to feel like gaps in their day.
Why Combining Drugs Multiplies the Risk
Mixing any of these substances dramatically increases the chance of a blackout, often at doses that would be manageable on their own. Alcohol combined with a benzodiazepine, for example, amplifies the suppression of the hippocampus because both drugs enhance GABA activity through slightly different mechanisms. The result is that much lower amounts of each substance can trigger complete memory loss.
This is especially dangerous with GHB or Rohypnol mixed with alcohol, where the combined sedation can rapidly progress from amnesia to respiratory depression. Z-drugs taken alongside even small amounts of alcohol increase both the likelihood of complex sleep behaviors and the depth of amnesia surrounding them.
Repeated Blackouts and Lasting Effects
There is growing evidence that frequent blackouts don’t just represent isolated memory gaps. They may cause or reflect cumulative damage to the brain’s memory systems. People who have experienced multiple blackouts perform worse on memory tests even when sober compared to heavy drinkers who haven’t blacked out. Two explanations exist for this pattern, and both may be true simultaneously: some people are inherently more vulnerable to memory disruption, and repeated episodes of heavy intoxication may damage the brain in ways that make future impairment more likely.
Animal studies support the idea that repeated binge-level exposure to substances worsens memory function over time. In human research, college students who engaged in repeated binge drinking episodes showed greater memory impairment during intoxication than students who drank similar amounts less frequently, suggesting that the pattern of use matters, not just the total quantity consumed.