Final answer:
The key instruction is to apply K9 Advantix directly to the dogs' skin, prevent ingestion by the dogs or contact with children or other pets until dry, and adhere to the prescribed dosage and frequency.
Step-by-step explanation:
The most important instruction for a veterinary technician to provide to an owner who has been given a prescription for imidacloprid (K9 Advantix) for flea control in adult dogs is to apply the medication properly and directly to the skin, ensuring the dogs cannot lick the area and to separate the treated dogs from other pets and children until the application site is dry. This is crucial because the medication could be harmful if ingested and needs to remain on the skin to be effective. In addition, it is important to follow the prescribed dosage and frequency as indicated by the veterinarian.