Humalog is a rapid-acting insulin used to manage blood sugar levels in people with diabetes. However, Humalog side effects can vary from mild to severe. Common side effects include hypoglycemia, characterized by symptoms such as dizziness, sweating, and confusion. Other Humalog side effects may include injection site reactions like redness and swelling, as well as weight gain and low potassium levels. It's essential to monitor for these side effects and consult a healthcare provider to ensure the safe and effective use of Humalog.