Mixing energy drinks with alcohol could worsen the negative effects of binge-drinking, according to new research.
Scientists have found that the combination could be responsible for making drinkers less able to engage in social communication and reduce their ability to feel fear.
They claim this goes against the aim of many people, who mix energy drinks with alcohol to counteract the sedative nature of alcohol by tricking them into feeling more awake and less drunk than they actually are.
Researchers from the University of Portsmouth and the Federal University of Santa Maria in Brazil tested the effects of taurine, a key ingredient of many energy drinks, and alcohol on social and fear responses in zebrafish.
They found that taurine seemed to increase the fear-reducing properties of alcohol, but also affected social communication.
Co-author of the study Dr Matt Parker, senior lecturer in behavioural pharmacology and molecular neuroscience at the University of Portsmouth, said: "The effects of mixing alcohol and energy drinks is yet to be established.
"This study is the first to show that the two together may be exacerbating some of the negative effects of binge-drinking - that is, reduction of fear and problems in social communication while intoxicated, which collectively increase the risk of fighting, violence and participation in risky behaviours."
In the study, published in the Journal of Psychiatric Research, the researchers tested how taurine and alcohol, at volumes reflecting levels that would induce moderate human intoxication, affected the behaviour of 192 zebrafish.
The fish that were exposed to both alcohol and taurine had fewer interactions with other fish in the shoal compared with those exposed to water alone or just alcohol.
These fish also showed more "risky" behaviour, spending more time in an area close to a predator than other groups.
Dr Parker said: "Binge-drinking and general alcohol misuse is a key problem in the UK and across the world, with the numbers of hospital admissions resulting from illness or injury following intoxication costing the NHS millions per year.
"Alcohol reduces our inhibitions, and in low doses can cause relaxation and euphoria.
"However, in higher doses this low inhibition can cause problems with fighting or risky behaviour.
"Zebrafish have similar biological and behavioural responses to alcohol, and are a highly social species, making them ideal for studying the effects of alcohol on behaviour.
"Here, we found that the addition of taurine, an ingredient in many 'energy' drinks, appears to exacerbate risky choices in zebrafish, as well as reducing their social cohesion.
"Taken together, these data appear to suggest that mixing alcohol and taurine might be a factor in increasing some of the negative effects of alcohol.
"People should be aware that drinking energy drinks in combination with alcohol may impair their judgment, and should do so with caution."
A spokeswoman for the British Soft Drinks Association said: "This study, carried out on fish, runs contrary to the body of evidence in this area and - as pointed out by the co-author - does not prove cause and effect.
"The European Food Safety Authority (EFSA) latest opinion confirms the safety of energy drinks.
"In fact, EFSA's meta-analysis of literature - the largest of its kind to date - found there to be no 'masking' effect of caffeine when combined with alcohol and states that alcohol consumption is unlikely to exacerbate the effects of caffeine on the central nervous system.
"EFSA concluded that studies linking consumption of energy drinks with alcohol and increased 'risk-taking' behaviour, were either cross-sectional or retrospective and did not allow a causal role to be attributed."