Some metal brackets and archwires contain the chemical element nickel, which is an allergen.
Nickel allergy affects 11% of women and 2% of men. These figures refer to allergic reactions of the skin. In the mouth, such a reaction is extremely rare. Even if a patient has a marked skin reaction to nickel, they are unlikely to react in the lining of the mouth as well. The frequency of nickel allergy inside the mouth is 0.1–0.2%. It's thought that a much higher concentration of the element in the mouth is needed to trigger a reaction in the oral lining. Certain conditions in the mouth encourage sensitivity: corrosion of metals by saliva, friction between metal surfaces, acidic food and drink (pH), genetic predisposition, and the use of fluoride products, which cause the metal to release more nickel.
The symptoms of an allergy in the mouth to metal components and braces are: a burning sensation, swollen gums, a rash – redness of the lining of the mouth, angular cheilitis (sores at the corners of the mouth), a skin rash around the mouth, loss of taste, a metallic taste, and tingling along the sides of the tongue.
The diagnosis is confirmed by an allergist with a skin allergy test.
If an allergy is found during treatment, the first step can be to replace the archwire, since some archwires have a high nickel content but there are alternative materials, such as steel. If the patient continues to show signs of allergy, it's advisable to remove everything metal from the mouth and continue treatment either with braces made of a different material or with clear aligners.
This answer is for general information. Your own case needs an examination by an orthodontist.