I believe alcohol quite effectively shuts down two mechanisms in the brain that allow for effective reasoning... I can't remember quite how so this is from Google...
"Ethanol disrupts the amount of chemical messengers in the brain. Specifically it screws around with glutamate and gamma-aminobutyric acid, or GABA for short. When you drink, alcohol goes from your bloodstream to the brain. There, it amplifies the effects of GABA, a brain chemical that prevents some neurons from sending messages to each other. Think of it as a kind of silencer, keeping neuron "noise" down so that your brain can focus on the most important signals. If your brain were a theater, GABA would be the "shusher" responsible for quieting down those not on stage.
GABA does its shushing by controlling the flow of negatively charged particles, called ions, through the tiny channels on brain cells. Too many negative ions in a brain cell shut it down. So when GABA wants to shush a brain cell, it lets the ions flood in. When alcohol comes along, it floods the brain with GABA, which results in an increased flow of brain-cell-shushing ions. (This is why you can't drink if you're taking drugs like Xanax or Valium, which crank up the amount of GABA in your brain. It'd be too much sedation for your body to handle.)
Alcohol also blocks glutamate, a molecule that typically activates neurons, so you get a doubly inhibitory hit. (Glutamate is a gatekeeper, like GABA, but it lets in positive ions that get brain cells to send messages to each other.) So when you drink alcohol, you're essentially turning off two systems of your brain"
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The most poignant experience I've had in this respect was after my first experience with what I believe was quite a potent hallucinogenic. At the beginning of the experience I was acutely aware of the changes to my perception, and despite knowing that they weren't 'real' it takes a lot to override what your brain is trying to tell you - which felt bizarre - like being cognisant of your impairments. Which, for me at least, is unlike alcohol... I mean, I know when I'm tipsy, drunk, battered etc... but I don't think you have the ability to interrogate your impairment when you're drunk, or rarely do. The hallucinogens were like being drunk and sober at the same time - this was a very odd sensation and difficult to accept for me. As the journey continued, I then made the mistake of swigging neat rum from the bottle (mostly out of habit) --- Where I'd been able to rationalise (to a point) the feelings and experiences I'd had earlier, the more drunk I got the less I could do that and the more I just fell into the experience. At the start, I'd written on a piece of paper in front of me, "There is no one here with you"... because I felt convinced a friend and his partner were in the house (despite knowing they were not). By the end of the bottle, my face was planted on the desk, I was having to think about breathing and believed that when I held my breath, time would stop. I was also confused about being stuck in a time loop because the same hook from Chris Isaacs "Wicked game" had been on repeat in my ears for what seemed like hours. The scribblings on the paper by the end of the night were all messages to people I thought were reading them. One upside (unrelated to alcohol probably) was that I may, for the first time, have seen colours my biologically colour deficient eyes had never seen - I gazed at my computer monitor in wonder at the Facebook logo thinking I'd never seen a blue like that before.
To me it was very noticeable that the introduction of alcohol removed my ability to logic or reason my way out what is effectively a dangerous state of confusion.... and having said that, I do not recommend it to anyone. Being aware that you are not in control of your mind is actually quite scary... with alcohol you know (hopefully) that your mind's okay - albeit possibly starved of oxygen and lacking certain controls - it's just you just can't seem to control your body properly... adding to that, a couple of times, for at least a year later, I'd wake up in the night, in a similar state of confusion, with the same paranoid feelings, and the same feeling of being aware you are not in control. That ain't nice, and the journey was not worth the risk.