Depends on how you use your insullin. My doctor gave me an insulin to carbohydrate ratio to use. I use 1 unit of insulin to every 12 grams of carbs I eat. The less carbs I eat, the less insulin I use. I have a very physical job for four hours a day and I don't even need my Humalog when I eat lunch. My pen generally lasts me the whole month. My Humalog is only good for 28 - 30 days. As far as I know, once your insulin pen is opened, that's how long most of them last no matter how much insulin you use.
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