Never use insulin if it looks abnormal. Regular, lispro, aspart, and glargine insulins are clear. If you use clear insulin, always check for any floating particles, cloudiness, or change in color. This could be a sign that your insulin is contaminated or has lost its strength.

Other types of insulin come as suspensions. This means that the material is not completely dissolved, and you might be able to see solid material floating in liquid. However, it should look uniformly cloudy. If you are using NPH, check that your insulin is free of any large clumps of material. Do not use any insulin if you see chunks of material floating around. These changes could mean that crystals or aggregates are forming and the insulin is spoiled or denatured. This can be caused by too much shaking of the insulin bottle or storing insulin at temperatures that are either too hot or too cold.

Dear Sir

Thank you for the very practical question, most people with diabetes are storing their insulin out of the recommended range. Precipitates may indicate loss of activity or even contamination by bacteria, however flakes may not harm but they can block a needle.