Just as your body can build up an immunity to certain antibiotics and other drugs, insulins can lose their effectiveness. There are different formulas and they all work in slightly different ways. You should ask your doctor if there is an insulin similar to the Novorapid that is less expensive. I am, however, confused by his choice. The Mixtard is a combination of fast acting and slow release insulins, which cover you throughout the day. He has changed you to three injections of fast acting insulin which won't cover you throughout the night. Do you also take Lantus once a day and use the Novorapid at meals?? How have your morning blood sugars been since this change? Do you wake up with highs in the morning? It has been my experience that unless you are on an insulin pump, which supplies your body with a stead stream of fast acting insulin, you need something long acting to keep your Blood sugars even between meals and during the night. Let me know how you are doing and about whether or not you are taking Lantus as well.