Nickel sulfate (Nickel(II) sulfate, NiSO4) is a yellow, green, or blue crystalline inorganic compound that produces toxic gases when heated. Nickel sulfate is used as a chemical intermediate in electroplating and the production of other nickel compounds. Exposure to this substance can cause severe dermatitis, skin and asthma-like allergies, and affect the lungs, kidneys, gastrointestinal tract, and nervous system. It is worth noting that Nickel sulfate is a known carcinogen that increases the risk of lung and nasal cancer.

The most common side effect of Nickel sulfate is allergic reaction. Nickel is the most common sensitizing hapten in patients who routinely undergo patch testing for suspected allergic contact dermatitis (ACD), with a clear predominance in the female population. In addition, by evaluating the difference in positivity of 2.5% and 5% Nickel sulfate patch tests, it was shown that of the 205 patients tested, 33% were positive (+, ++, or +++) to at least one concentration of Nickel sulfate, 20% were positive to 2.5% Nickel sulfate, and 31% were positive to 5% Nickel sulfate. Patients were 6.5 times more likely to have a positive reaction to 5% Nickel sulfate than to 2.5%. Therefore, using 5% nickel sulfate as the standard allergen concentration for patch testing may be more accurate.