Hello
I hope Sadie is doing well after her operation, you don't say what the operation was for. As you are probably well aware Previcox is an anti inflammatory and it lasts 24 hours, usually vets give an injection on the day of the operation and then medication starts afterwards, I know you feel disappointed in your vet but please please have a chat with them as labels with written instructions are a must and it may just be an over sight. Previcox can be given more than once a day and as you already know at a higher dose but there maybe a reason that your vet has given you a low dose, such that Sadie is an older dog, she is on other medication or that there maybe underlying kidney or liver problems so wouldn't be able to comment on doses. It is better to give anti inflammatories with food so just a small amount of food so you know Sadie has had it. Please speak to the vet
Give Sadie a cuddle from me and I hope everything goes well
Sarah