A contra-action can take place during or after a treatment. A common contra-action associated with eyelash extensions is an allergic reaction, which can occur as a result of ingredients within the adhesive. This is the purpose of the sensitivity patch test and consulting them about any allergies they suffer from beforehand. Allergic reactions can occur at any time and without warning, even after a negative patch test result.

If a client does react to any products during treatment, remove them immediately with water and apply a cold water compress, before making a record of the reaction and action taken. Skin sensitivity tests with the products used must be carried out later in order to identify the allergen.