As loveable as it is, garlic is off the table for many sensitive foodies. Some are allergic, which is not very common. It's an immune system reaction that causes your body to go into a fight-or-flight response. Typically that involves various parts of the body that seem unrelated to eating, like the chest, head or skin. 

Many more of us just can't stomach it, or much of it. As in, it causes upset tummies, of the usual variety: bloating, gas (top and tail), heartburn or nausea. Food intolerances like this mean your gut struggles to digest, or reacts to a substance in a food or food group. In the case of garlic, the most likely culprit is called an oligosaccharide. The thing of interest about that is that it's the 'O' in FODMAP, an acronym for the substances in certain foods that commonly cause intolerances. 

Garlic is part of the same food family ('allium') as onions, leeks, shallots and chives, a renowned dysfunctional family rich in those irksome O's. So this is one to actively test for, keeping in mind that it may not be a yes/no answer - you may only be intolerant to eating more than you can calmly digest. And if you know you react to one of them, look out for an intolerance to others. Also be warned that this family gets around! As cooking staples, they are near ubiquitous in packaged, ready-made, canned and processed foods.