It's not advisable, and it's not always safe, but passing out drunk happens sometimes when people over-indulge – having so many drinks their bodies just can't handle it, leading them to sink into bleary unconsciousness.

The hangover could be far worse than a sore head the next morning, scientists now report in a new study. A massive review of previous research examining the drinking history of over 130,000 people suggests that getting blind drunk stands to double people's risk of developing dementia later in life.

While the associations between excessive alcohol consumption and dementia are well documented, there nonetheless remains much we don't know about how drinking alcohol contributes to the biological mechanisms behind cognitive decline, as seen in conditions like Alzheimer's disease.

Even so, the links have been observed in numerous studies, but while the territory may be well explored, that doesn't mean there aren't new ways of approaching the data.

One significant limitation in existing studies, which tend to analyse effects in terms of average alcohol consumption levels over time, is a lack of consideration of the drinking patterns hidden behind the averages.

In terms of the downstream effects on your health, for example, there's probably a big difference between drinking 14 alcoholic drinks in a fortnight if you have them all in one night, as opposed to spacing them out over 14 separate nights.

"Consumption of high quantities of alcohol in a short time can lead to neurotoxic blood levels of alcohol, although such episodes are not fully reflected in average consumption levels," a research team led by epidemiologist Mika Kivimaki from University College London explains in the new report.

"Thus, both heavy and moderate levels of overall consumption may be combined with excessive drinking episodes leading to acute central nervous system effects, such as loss of consciousness."

According to Kivimaki and colleagues, the legacy of such neurotoxic effects from alcohol-induced blackouts hasn't been comprehensively examined in the context of risk factors for dementia, so the researchers sifted through data from seven previous studies, measuring alcohol intake in cohorts from the UK, France, Sweden, and Finland, totalling 131, 415 participants all up.

Not all of those participants reported drinking alcohol to the point of passing out, but over 96, 000 said they had experienced such a loss of consciousness, and about 10,000 said they'd experienced it in the previous 12 months. 

When those participants were followed up on, a disturbing trend emerged.

"Loss of consciousness due to alcohol consumption was associated with double the risk of subsequent dementia irrespective of overall alcohol consumption," the researchers explain.

"Those who reported having lost consciousness during the past 12 months had twice the risk of dementia [compared to] moderate drinkers who had not lost consciousness."

The researchers say this roughly doubled risk was evident for all-cause dementia, plus early-onset dementia, late-onset dementia, and Alzheimer's disease, and could be seen in both men and women, and in older and younger participants.

The specific level of risk (hazard ratio) differs slightly in each subset, but on the whole, the team says the elevation of dementia risk was about two-fold in drinkers who reported having passed out, even if they were only otherwise moderate drinkers (defined in the study to mean less than 14 units of alcohol a week, per current UK guidelines).

When comparing such moderate drinkers to heavy drinkers (individuals consuming more than 14 units in a week), heavy drinkers were about 1.2 times more likely to develop dementia later in life.

As with any observational analysis like this, there are numerous limitations to be aware of due to the way the data are collected, which the researchers acknowledge.

But perhaps the most important to thing to be aware of is that research like this can only ever highlight correlations in the data, not demonstrate causation of whatever's occurring.

In other words, we can't conclude that people who drink to the point of losing consciousness are necessarily seeding their future dementia somehow; all we can confirm is that people who report having such blackouts due to alcohol consumption, do seem to be at significantly higher risk overall.

There could indeed be a biological mechanism involved, the researchers speculate.

"Ethanol is neurotoxic, crosses the blood-brain barrier to reach neurons directly, and, in high concentrations and with its metabolite acetaldehyde, can initiate pathologic processes leading to brain damage," the authors write.

"Neurotoxic insults may be due to release of large amounts of glutamate, which overstimulates the brain and results in excitotoxic effects via excessive N-methyl-d-aspartate receptor activity, which damages or kills brain cells."

Alternatively, the researchers point out that episodes of particularly heavy drinking could be contributing to the development of other health conditions linked to dementia, such as liver and kidney disease, diabetes, and coronary heart disease, among others.

What we don't know remains legion, but what we do know is yet valuable. Let's face it: drinking to the point of oblivion was never a good idea – for anybody, ever. But now we can be surer than ever that it's a particularly bad one.

The findings are reported in JAMA Network Open.