The TST is a simple and safe test and side effects are uncommon. The side effects are mainly related to the size of the reaction and include swelling, redness and itchiness that may take a few weeks to clear.Very strong reactions are uncommon but may result in a painful swelling of several centimetres in size, blistering or ulceration. This will heal with almost no scarring. The effects of the TST are localised to the site and more general reactions are extremely rare.