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Causes of low blood sugar in diabetes

It is always important to consider the possible causes of low blood sugar. In particular, it is necessary to investigate the underlying reasons for recurrent episodes of low blood sugar in order to anticipate and prevent similar situations in the future.

There are numerous reasons for occasional low blood sugar in insulin therapy. Often, multiple factors may contribute, and the exact cause may also remain unclear. However, it is always necessary to investigate the underlying reasons for recurrent episodes of low blood sugar in order to anticipate and prevent similar situations in the future. The situation can be best assessed through the use of glucose monitoring sensors.

The blood sugar level of a person with diabetes drops lower than normal (under 4 mmol/l) when there is more insulin in their body than necessary. Low blood sugar can be related to the following situations:

  • You have overestimated the amount of carbohydrates in a meal, resulting in the insulin dose being too large and your blood sugar dropping too low after the meal.


  • If the basal insulin dose has, for one reason or another, been increased too much, it can easily result in the blood sugar level dropping in the early part of the night or in daytime before meals or during exercise.


  • If you have accidentally injected too much insulin or the wrong type of insulin, for example, if you accidentally took bolus insulin instead of basal insulin and vice versa.

If you administer rapid-acting insulin only after a meal, the effective time of insulin may not align properly with the impact of carbohydrates from the meal. As a result, your blood sugar levels may initially rise too high (1-2 hours after eating) and when the meal's blood sugar-raising effect diminishes (2-4 hours after eating), the insulin's effect may still persist, leading the blood sugar dropping too low.

Insulin is absorbed into blood circulation at different rates depending on the injection site. Absorption is fastest in the abdominal area and slowest in the buttock region. Due to the variability in absorption depending on the injection site, it is advisable to aim for consistent use of a specific insulin product in a certain area. However, it is important to regularly rotate the injection sites within that area, such as switching sides of the abdomen or alternating between thighs.

Mealtime insulin is usually recommendable to inject in the abdominal area.

Basal insulin is usually recommendable to inject in the thighs or buttocks. If the subcutaneous layer of fat in your thighs is thin, you can regularly inject basal insulin also in the side of your abdomen.

The condition of the injection site has an effect on the absorption of insulin. Repeatedly injecting in the same place can cause tissue changes, which slow down the absorption of insulin. If there is tissue damage in a particular area, the effect of insulin may be more pronounced when transitioning to a new undamaged injection site.

Higher temperature during a heatwave, in a sauna or bath, for example, makes insulin faster to absorb. So does faster circulation i.g., during exercise.

New tablets or intestinal hormone injections for type 2 diabetes do not as such cause severe hypoglycaemia without insulin therapy. Of the older tablets, sulphonylurea and glinides work in a formulaic way, which may lead to a the blood sugar level dropping too low, if the dose is incorrect relative to a meal or exercise or if their effect has increased for some reason.

Endurance exercise consumes energy, increases the effect of insulin and reduces the blood sugar level. That is why it is a good idea to prepare for it by bringing some extra carbs with you and/or reducing the bolus insulin dose and, if necessary, even the basal insulin dose acting during the exercise.

After a long exercise session, the risk of hypoglycaemia is also higher the following night.

When using mealtime insulin, it is recommended to accurately calculate the consumed carbohydrates because if the injected insulin is proportionally higher than the consumed carbohydrates, it will result in low blood sugar.

However, carbohydrate counting is not always feasible, and some, especially elderly individuals, may dose mealtime insulin as fixed doses, meaning the same dose is taken at the same time for the same meal. If the amount of carbohydrates in the meal is lower than usual, the blood sugar may drop excessively. Therefore, when consuming fewer carbohydrates than usual, it is necessary to reduce the dose of mealtime insulin also when using fixed doses to avoid excessive insulin impact.

The insulin therapy of type 2 diabetes may entail quite large basal insulin or combination insulin doses. In this case, the doses are designed on the basis of person's usual meal sizes and amounts of carbohydrates. If a meal is late or the amount of carbohydrates is smaller than usually, the blood sugar level can easily lower too much.

When blood sugar drops, liver releases sugar stored in it into blood circulation. Alcohol prevents sugar from being released from the liver, increasing the risk of low blood sugar.

The biggest risk of low blood sugar is in the early hours or morning after consuming large quantities of alcohol.

In a drunken state, it is harder to spot the symptoms of hypoglycaemia and you can't safely take care of yourself.

Heavy alcohol use may cause severe hypoglycaemia and an insulin shock.

The underlying factor behind recurring low blood sugar may also be another medical condition contributing to the insulin requirement. Such conditions include hypothyroidism, adrenal insufficiency and kidney failure.

Updated 30.9.2023