Hypoglycemia (low blood sugar)
The most common and most urgent side effect of insulin is hypoglycemia - blood glucose dropping too low, which can happen if a dose doesn't match food intake, activity, or timing. Early signs often include shakiness, sweating, rapid heartbeat, irritability, hunger, and difficulty concentrating. Left untreated, it can progress to confusion, seizures, or loss of consciousness.
If blood sugar is low
A common approach is the "15-15 rule": consume about 15 grams of fast-acting carbohydrate, wait 15 minutes, and recheck blood glucose, repeating if still low. Severe hypoglycemia (confusion, unconsciousness) is a medical emergency - glucagon or emergency services may be needed.
Other possible side effects
- Weight gain - insulin helps cells take up glucose, some of which can be stored as fat.
- Injection-site reactions - redness, swelling, or itching where insulin is injected.
- Lipohypertrophy - lumpy fatty tissue from injecting the same spot repeatedly.
- Allergic reactions - rare, but possible with certain formulations or preservatives.
When to contact a doctor
Frequent lows, unexplained highs, injection-site problems that don't improve, or any signs of a serious allergic reaction (swelling of the face/throat, difficulty breathing) all warrant a call to your care team.