An auxiliary unit (100) for sanitising treatments can be fitted on dental units (R) comprising: a fluid supply line (1) connected, at one end, to the general water mains (2) and, at the other, to the dental unit by means of a first branch (3) which supplies at least a series of instruments - patient (4) and an air supply line (6) connected at its infeed to a source (7) of compressed air and at its outfeed to the dental unit; the unit (100) comprises a body(8), external to the dental unit, supporting at least one container (9) of disinfectant / sterilising fluid which can be connected, by means of a first conduit (10), to the fluid supply line; second means (11) of interception positioned on the fluid supply line (1) and designed to allow closure of the fluid supply at least in the first branch (3) during the treatment using the disinfectant / sterilising fluid; and means (12, 13) for preparing and implementing the treatment, acting at least on the first conduit (10) and on the second means (11) of interception, so as to allow the treatment fluid to pass through the first branch (3).