Local reactions to insulin injections are uncommon, when they do occur they are more likely to be due to sensitivity to the added preservatives (metacresol, phenol or methylhydroxybenzoate) than to the insulin. A trial of insulin with a different preservative may solve this problem. Formal identification of the preservative is possible using preservative-only preparations available from the manufacturers. Cold-sensitivity urticaria should also be considered if the patient develops a local reaction on using a vial directly from the refrigerator.

Mixing Insulins
In order to maintain uniformity, clear insulin (short-acting) should be drawn up into the syringe before cloudy insulin (intermediate or long acting). This strategy prevents contamination of the short acting insulin by the longer-acting insulin and eliminates the possibility of converting the short acting insulin into a longer acting form.

NPH insulin can be premixed with short acting insulin in the same syringe or vial without altering the absorption profile of the short-acting insulin. There are a variety of premixed NPH short-acting insulins with different ratios on the market.

Lente insulin can be safely given together with short acting insulins in the same syringe providing the injection is administered immediatly after mixing. Lente insulin should never be premixed and stored with short acting insulin in the same syringe or in a vial because of conversion of the shor acting insulin into a longer acting form. Insulin form different manufacturer should not be mixed as there may be differences in the buffers used.
NPH and lente insulins should never be mixed in the one syringe or vial.

Storage Conditions

• insulin is stable at room temperature for several weeks providing there are no extremes of temperature.
• unsused vials should be refrigerated but never frozen.
• insulin may lose its potency after opening the vial or when exposed to high temperatures.
• insulin vials should be discarded after 3 months of opening if kept refrigerated and after 1 month if kept at room temperature.
• penfill cartridge should be discarded after 3 months of inserting into the pen if kept refrigerated and after 3 weeks if kept in the pen at room temperature.
• the manufacturer's expiry date should be adhered to.