In the US, combination products are categorised into drug/biologic or device regulatory schemes based on the primary mode of action (PMOA). The PMOA can be straightforward for certain combination products. It could be either a device action or a drug/biologic action. However, the determination of the PMOA for complex multimodal and novel combinations are more complex. This article will review a case of a borderline drug/device combination, with a disputed PMOA, and apply a simple regulatory framework to determine the appropriate PMOA. The article details how the FDA determines the PMOA for combination products and the importance of early determination.