After drinking alcohol from time to time, you start to develop alcohol tolerance. This means that you need to consume more alcohol to have the same effect. The alcohol tolerance can be functional, when brain functions adapt to compensate for the effects of alcohol, physiological and behavioral one, and metabolic, meaning the rate at which the body can process and eliminate alcohol.
The factors of metabolic tolerance can explain why people get sick after drinking. It is known that alcohol is absorbed primarily from the stomach and small intestine, and then metabolized in the liver, by an enzyme. If the body doesn't have enough of this enzyme, the alcohol cannot be metabolized. There is also a second enzyme that takes part in this process. People that are lacking the second enzyme experience facial flushing, sweating, and they get sick after drinking alcohol, even small amounts. Furthermore, women have higher body-fat percentages, and this has a role in metabolizing alcohol differently, meaning that a woman will have a higher concentration of alcohol in her body than a man, after drinking the same amount of alcohol.
Other reasons why people get sick after drinking alcohol are an irritated stomach because it is known that alcohol irritates the esophagus, stomach, and intestines, and increases the production of gastric acid, resulting in nausea or vomiting; sulfites from the wine, if that person is allergic to sulfites; an empty stomach; age because as the drinker is older, his body is processing the alcohol slower; mixing the alcohol with medication or several medical conditions. If you already have a health condition, like ulcers, high blood pressure, severe acid reflux, you have to stop drinking. Considering that alcohol affects most internal organs, already having a problem at one of them, it will make you even sick when drinking.