If you are one of the few people who are truly allergic to onion, eating can be difficult for you as onion/onion salt/onion powder is found in so many foods!
If you are truly allergic to onions, your body will take protective measures such as releasing histamine which can cause symptoms of a reaction. These symtoms can include hives, rash, tingling or itching of the mouth, swelling of the lips, face, tongue or throat, nausea/vomiting, stomach pain, cramping, difficulty breathing, nasal congestion, gas, dizziness or lightheadedness, true anaphylaxis (which is rare).
Symptoms can occur immediately or up to several hours after ingesting onions. Symptoms can be mild or severe.
Being sensitive to onion (intolerance) is more common than a true allergy. Intolerance of nonallergic food hypersensitivity is caused by an inability to process or digest certain foods. This is not an immune (or IgE mediated) reaction which deems it to not be an "allergy".
Intolerance vs Allergy? Sometimes it can be difficult to tell. The results of intolerance if onion is ingested would be much less severe and typically not life threatening.
Symptoms typically resolve once the onion leaves your system. Should you have a severe reaction involving difficulty breathing or severe vomiting or dizziness, you should seek help immediately. In rare cases, if you are truly allergic you may experience symptoms for days after exposure.
If you seek the care of an allergist and are skin tested and deemed to be allergic to onion, you need to be aware of cross-reactivity from other food and plants. These include garlic, chives, scallions and shallots. Mugwort which can be found mostl commonly in asian tea is also included with these substances.
Avoiding these substances can be quite a challenge and will require label reading and vigilance on your part! You should avoid deli salads and salad bars, flavored broths, canned soups and sauces, crackers, frozen pizza, most frozen entrees, flavored cheeses. If you find something that you are not sure about, you can try contacting the manufacturer.
There are plants that are in the same genus as onion (the allium genus) that may cause allergic symptoms upon exposure. These include all alliums, wild onions, wild chives, most lillies, daffodils, and tulips.
The best treatment is avoidance. Should onion be ingested you may take oral antihistamines to lessen the severity of the reaction. You can use aloe vera or hydrocortisone cream to calm down itchy hives or rash. Your doctor may prescribe an Epi-Pen which is used to treat severe reactions such as anaphylaxis and a prescription bronchodilator if you have had symptoms of difficulty breathing, wheeze etc.
It is always best to check with your physician if you suspect that you have an allergy to something that causes symptoms of concern. Vigilance and avoidance will help keep you safe.