Might be handy to actually say what "terrible negative effect" is has on you

The classic sign of an “allergy” is “alcohol flush reaction”, more common with certain Asian ancestry but could be anyone. It’s actually an inability to metabolize alcohol and can be much worse than just a red face, rather than an allergy. look that up and if it fits, you should avoid alcohol

https://en.wikipedia.org/wiki/Alcohol_flush_reaction

(As an idiot in college I was always entertained by my roommate turning bright red and acting loopy on less than one beer. Now I know to be concerned)

If your interest is in identifying what component of beer is triggering your reaction, it might be helpful to make friends with a homebrewer or your local small brewery. They can provide you with the individual ingredients (hops, yeast, wort) so you can test which of them gives you the bad reactions.

Once you figure that out, you can experiment with seeing if there's styles that don't affect you as much. If it's hops, there are styles that go much lighter on the hops, or might use a strain that doesn't cause as much of a reaction for you. If it's yeast, try some non-traditional yeasted beers - lagers use a completely different type of yeast than standard ales, as do Weiss beirs (like a heffeweissen) and sours (wild ales) - or just cut out the yeast entirely and do seltzers. If it's wort (the steeped grain water where the sugar comes from that the yeast converts to alcohol) try some gluten free beers to see if it's a gluten allergy causing you problems. If gluten free doesn't make a difference, you're likely better off avoiding beer (and whiskey, effectively unhopped and distilled beer).

The difference, not "the door difference"

I used to always feel tired after drinking beer. I’d be going to sleep after 3-4 beers and glurking up foam - then it was like I had a hangover before I went to bed. I noticed that I felt more clear-headed with 5-6 cocktails, shots or glasses of wine than 3 beers.

It turned out this was because of celiac disease, so it was a reaction to the gluten in beer. So my question is do you mean just beer or all alcoholic drinks?

I'm not sure whether there actually is an allergy against alcohol. Generally, an allergy is an exaggerated response by your immune system to a substance. Basically, your body thinks, some substance (mostly proteins) is part of a pathogen (like a hull protein of a bacterium) and responds accordingly. There are different types of allergies depending on what part of your immune system gets activated.

The adverse effects of alcohol on the other hand, are mostly effects of the poisonous byproducts of intermediate products of alcohol metabolism (the process in which your body gets rid of the alcohol).

Of course, you can be allergic to another compound of an alcoholic beverage, like some proteins from the malt used to make beer or have another kind of reaction, which isn't an allergy to other compounds.

I have wicked hangovers with anything malted (malted beers, hard seltzer, etc). Just one can will give me a headache the next day. Pay attention to which beers or other drinks you're consuming, or are you saying that ALL BEER of any kind puts you over the edge? And like someone else said, you need to define your symptoms.

I sometimes have sneezing fits if I'm drinking high hops beer e.g. IPA, ales etc. Don't suffer from this with spirits and lighter beer e.g. lager. So anecdotally I put this down to a slight allergy to hops rather than ethanol as such

Basic BAC calculator.

Pace yourself when drinking so that you are metabolizing the drinks fully before starting a new one. If you still experience hangover symptoms the next day despite being sober, you may have an issue with metabolizing but probably aren’t allergic, since being allergic outright to alcohol would result in more immediate symptoms.