My brother had to take insulin while he was on strong chemo for 4 months then returned to his normal controls when he was done. Those extreme highs and lows are dangerous, especially when your system is being taxed already by herceptin. Please talk to whatever doctor you normally see about your diabetes to get this under control, it might only have to be temporary, but during herceptin your body needs all the love you can give it. Do you normally see an endocrinologist for your diabetes?