Safety of concomitant use of veterinary products is of clinical interest. A series of studies was performed to evaluate the chemical compatibility and short term dermal and systemic safety of an imidacloprid/ flumethrin collar (Seresto ® / Foresto ® , Bayer) used concomitantly with spot-on or tablet formulations. Chemical compatibility was evaluated in-vitro (study reference A) on collar pieces, followed by two small, non-controlled clinical studies (study reference B) in both, cats and dogs. The studies showed, that certain solvents affected the collar in-vitro, but not in their marketed formulations.Dermal and systemic safety of different spot-on or tablet formulations was first evaluated in a small, non-controlled clinical study (study reference C) in cats and dogs, via clinical observations only, followed by controlled clinical safety studies of concomitant use with imidacloprid/ moxidectin (Advocate ® / Advantage ® Multi, Bayer) in dogs and cats (study reference D) and emodepside/ praziquantel (Profender ® , Bayer) in cats (study reference E), assessing safety aspects by clinical observations and statistical analyses of hematology and clinical chemistry parameters compared to baseline values and between treated and control groups. S55 Ectopar asitEsDermal safety findings over all clinical studies (study references B to E) matched those already described for the respective products and included transient cosmetic changes (oily hair and crystal formation) at the site of spot-on application and broken hair, transient alopecia and skin alterations at the site of collar application. There were no indications of these findings aggravating under the conditions of concurrent use. There were no systemic safety findings of clinical significance in any of the clinical safety studies (study reference C to E). Assessment of blood parameters revealed some deviations from baseline levels and from the reference range in dogs as well as in cats, but no clinical relevance could be deduced. Hematology and clinical chemistry results confirmed the safety of the concomitant treatment. It is concluded that Seresto ® is chemically compatible with solvents used in major spot-on formulations on the market and is dermally and systemically safe for adult dogs and cats when used concomitantly with Advocate ® and Profender ® spot-on formulations.