Beer isn't for everyone. That's fair enough. However there are some people that get physically ill from it almost immediately.
Are there specific ingredients in beer that can cause this? Or is it dependent on the consumer?
If the ingredients cause this, is there a way to nullify/reduce the effects?
(Assuming you are referring to "plain" beers, not beers with weird ingredients)
The only allergen that is present in normal beers is malt that contains gluten. Gluten intolerant people can therefor react to beer very aggressively.
Next, I would guess the alcohol. Some people have violent reactions to any form of alcohol.
Then, maybe the bitterness or just the flavour. The people may be sensitive to bitterness, or just really do not like the taste of (that) beer.
I have had beers that taste like baby vomit. It is VERY hard to not gag when you taste that!
You have to determine why the person reacts that way. If it is the malt, then get them a gluten free beer, if it is alcohol, then try an alcohol free beer, or just a cold drink, if it is the bitterness, then get a beer that is less bitter, if it is the flavour, then get a beer with a different flavour (fruit beer or a different style of beer that has other prominent flavours).
In general, beer only has 4 ingredients: grain, hops, yeast and water. Of those 4 ingredients, 3 can cause issues with somebody who is sensitive (everything but water). As Tom said, if a person has Celiac's Disease, they're going to be sensitive to the gluten that that is in barley and wheat, which is why some breweries are offering an alternative (usually made with sorghum). It's also possible that hops might be an issue for somebody sensitive. I have a friend who is allergic to hops. There are also folks who are sensitive to yeast. Both ale yeast and baker's yeast are Saccharomyces cerevisiae, so if somebody has trouble with yeast raised bread, they'll have trouble with beer too. A yeast sensitivity can be mitigated by filtering the beer, but not all of it is captured.
All that said, I thought that I was going to get sick before I tried a certain maple bacon doughnut beer and surprisingly, I really enjoyed it.
There is a condition caused by alcohol, called an "alcohol flush reaction". Some people (often from Asia) have a condition, not related to allergies, that makes them lack a certain enzyme that lets their bodies process alcohol properly. The condition apparently causes rapid intoxication and nausea after drinking only small amounts of alcohol, and I believe the reason for the sufferings are an overproduction of the same waste chemicals that causes the really bad hangovers (aldehydes). Anyhow, I wouldn't blame them for being abstinent.
There are a few common allergens in beer- hops are a fairly common allergen, and gluten intolerances or celiac's disease can cause physical illness. Yeast is also a common allergen.
Also, some people lack the ability to properly process alcohol- this often manifests by a single beer being enough to cause illness and vomiting.
