Insulin is a hormone produced by the pancreas that helps move glucose (blood sugar) from the bloodstream into the cells of the body. The cells break down glucose and convert it to energy. When the body cannot produce insulin or fails to respond properly to it, glucose builds up in the blood. This condition is known as diabetes.

Patients with type 1 diabetes produce virtually no insulin and depend on insulin therapy to survive. In patients with type 2 diabetes, either their cells are resistant to insulin or the body does not use it effectively. This allows sugar to build up in the bloodstream, which triggers complications of diabetes. When exercise, diet and antidiabetic agents are not sufficient to manage type 2, insulin treatment may be necessary to maintain normal glucose levels.

Insulin treatments help prevent glucose levels from going too low or too high. Untreated, diabetes can cause long-term health problems, such as serious nerve, blood vessel, kidney and eyeproblems.

Women with gestational diabetes, which can develop during pregnancy, also may need insulin injections because they generally cannot take antidiabetic agents.

A physician will examine a patient’s diet, exercise habits, physical condition and stability of glucose levels to determine the appropriate insulin therapy. Though patients can obtain most forms of insulin without a prescription, a physician should always be consulted when developing a dosage level and treatment regimen.

Syringe injections are the most common way of taking insulin. Other methods include insulin pumps, insulin pens, insulin jet injectors and inhaled insulin.

About insulin

When a person consumes carbohydrates, the nutrient is broken down into a simple sugar called glucose, which is absorbed into the bloodstream. Glucose is the body’s primary energy source.

Insulin is a hormone that helps move glucose from the bloodstream into the cells of the body. Insulin is produced by specialized beta cells in a part of the pancreas called the islets of Langerhans. When the body cannot produce enough insulin or fails to respond properly to it, the condition is known as diabetes.

The pancreas also produces a hormone called glucagon that acts contrary to insulin by raising levels of glucose in the bloodstream as needed.

Insulin levels can become too high or too low. In type 1 diabetes, the pancreas produces virtually no usable insulin. People with this condition depend on insulin injections to live. In people with type 2 diabetes, the cells are insulin-resistant, meaning they have difficulty absorbing insulin and, consequently, glucose.

In both cases, elevated levels of glucose in the blood causes symptoms of diabetes (e.g., excessive thirst, hunger, fatigue) and may lead to serious complications, such as nephropathy (kidney disease), retinopathy (eye disease) and cardiovascular disease.

About 12 percent of American diabetic patients (1.7 million) take insulin and antidiabetic agents, and 16 percent (2.1 million) take insulin only, according to the Centers for Disease Control and Prevention. The proportion of those taking only insulin has declined in recent years, and the number of those taking antidiabetic agents only or insulin and antidiabetic agents has risen.

Insulin treatments maintain glucose levels that are normal and that will not trigger symptoms. Though patients can obtain most forms of insulin without a prescription, a physician should always be consulted when developing a dosage level and treatment regimen. Insulin can be delivered via several different methods, including:

  • Subcutaneously (under the skin)
  • Intramuscularly (into muscle)
  • Intravenously (into a vein)
  • Orally (inhalation through the mouth to the lungs)

Insulin is not swallowed because stomach acid destroys it, though researchers are trying to create insulin pills that overcome this obstacle.

Optimal places to inject insulin include the abdomen, upper arms, thighs, hips and calves. Different locations on the body absorb insulin at different rates. The preferred site is the abdomen because the insulin is absorbed most consistently. The optimal sites after the abdomen are the arms, thighs and hips.

Each injection site should be about a half-inch from the location of the previous injection to prevent the erosion of fat beneath the skin (lipodystrophy). Rotating the injection site within the same site location will ensure that insulin is absorbed at a more constant rate. The same injection site should be used no more than once a month.

Insulin can be delivered via needle-and-syringe injections, insulin pen, jet injector or insulin pump. The U.S. Food and Drug Administration (FDA) and European regulators approved the first form of inhaled insulin 2006. Other methods are being developed.

The amount of insulin a patient requires depends upon factors such as diet, exercise and weight. Methods include:

  • Needle and syringe, or cartridge or prefilled pen injection. Insulin is injected into the fat just beneath the skin. Insulin pens use cartridges filled with insulin that are inserted into a device that resembles a pen. The insulin pen is convenient for travel and for use at school or work.
  • Insulin jet injectors. These devices use a high-pressure air mechanism to push a fine stream of insulin into the skin. Many who use this method say it is less painful than needle-and-syringe injections.
  • Insulin pumps. These small, computerized delivery devices are usually worn on a belt or inside a pocket. About the size of a pager, insulin pumps have a small, flexible tube attached to a thin needle. The needle is inserted into the abdomen and taped into location. The needle requires cleaning and reinsertion every few days. Pumps deliver a continuous flow of rapid-acting insulin. Surgically implanted insulin pumps are less readily available
  • Inhaled insulin. Powdered insulin is inhaled through the mouth with a special inhaler. The medication travels to the lungs and is absorbed into the bloodstream. Some people using inhaled insulin, especially those with type 1 diabetes, must still inject long-acting insulin.

It is crucial that patients properly store insulin and use it only if it is still good. Patients are advised to consult with their pharmacist or physician for specific instructions on insulin storage. Generally, insulin should be kept in the refrigerator and should not be frozen. Insulin that is frozen, heated or outdated should be discarded. Depending on the type of insulin, an appearance that is cloudy, clumped or crystallized may also be a reason to get rid of the medication. However, intermediate-acting and long-acting insulins are normally cloudy.

If refrigeration is not available, insulin should be kept in a dry place at room temperature and out of sunlight. Once it is opened, manufacturers generally recommend that insulin should be used within 28 days whether it is refrigerated or not. Insulin should not be kept in hot places such as the trunk or glove compartment of a car, or very cold places such as the freezer.

When traveling, be sure to keep bottles protected (such as wrapping them in clothing) and accessible (such as keeping them in carry-on baggage during air travel). The following rules are recommended to ensure safety:

  • Do not use regular insulin or Humalog if it becomes cloudy in appearance.
  • Do not use NPH or Lente insulin if it becomes clumped or crystalized or if the bottle becomes frosty.
  • When combining insulin, be sure to inject the insulin within five minutes of mixing.

Types and differences of insulin

There are more than 20 types of insulin products available in four forms, according to the U.S. Food and Drug Administration (FDA). These forms are:

  • Rapid-acting insulin. Known as Lispro (Humalog) or Novolog, it begins working within 15 minutes.
  • Short-acting insulin. Known as “regular” insulin, it takes 30 to 45 minutes to work.
  • Intermediate-acting insulin. Known as NPH or Lente (no longer available in the United States), it is mixed with a substance that causes the body to absorb the insulin more slowly. It begins to work within two to four hours.
  • Long-acting (basal) insulin. Known as Lantus or Levemir, it acts in a way that is more or less stable for the whole day.

The type of insulin that is best for a patient depends on several factors, including:

  • Onset of action. When the insulin starts lowering glucose (blood sugar) levels.
  • Peak of action. When the insulin provides the maximum effect on glucose levels.
  • Duration of action. How long the effect lasts.
  • Health requirements of the patient, which include glucose levels, ability to perform glucose monitoring, exercise, diet and other medications.

In addition, there are a number of factors individual to each patient that may influence the type of insulin appropriate for treatment, including:

  • Individual response to insulin. The period of time it takes for insulin to be absorbed and the length of time it remains active vary from patient to patient.
  • Lifestyle choices. Diet, alcohol use and exercise patterns all affect how the body processes insulin.
  • Level of comfort with multiple injections.
  • Willingness to check glucose level frequently.
  • Age.
  • Glucose management goals.

Insulin derived from animal pancreases first became available to diabetic patients in the 1920s. Genetically engineered synthetic insulin, which is similar to natural human insulin, has in recent decades largely replaced insulin obtained from cows and pigs.

The same source of insulin (human or animal) made by the same manufacturer should be used, to ensure consistency in insulin delivery and effect.

Conditions treated with insulin

Insulin is used to treat various forms of diabetes, including:

  • Type 1 diabetes. Patients with this condition are unable to create insulin, and depend on insulin injections in order to survive.
  • Type 2 diabetes. Patients with this condition have cells that are resistant to insulin. Over time, this insulin resistance can lead to a drop in insulin production. Sometimes, changes in diet and exercise can control the condition by themselves. In other cases, antidiabetic agents and/or insulin therapy may be necessary to keep glucose (blood sugar) levels normal or near normal.
  • Gestational diabetes. High glucose levels that can occur during pregnancy in previously nondiabetic women. Some women with this condition may need to use insulin to control their glucose levels and to avoid health problems. Pregnant women generally are not prescribed antidiabetic agents.
  • Latent autoimmune diabetes of adulthood (LADA). Nearly half of patients with LADA, which is generally categorized as a subtype of type 1 diabetes, need to take insulin within four years of diagnosis.
  • Maturity-onset diabetes of the young (MODY). Insulin may be prescribed to patients with this rare genetic disease, which is sometimes considered a subtype of type 2 diabetes.
  • Wolfram syndrome. This rare genetic disorder involves insulin-dependent diabetes mellitus, diabetes insipidus, deafness and damage to the optic nerves.

Conditions of concern with insulin

Certain conditions may affect insulin treatment, such as:

  • Allergic reactions to:
    • Insulin or other drugs
    • Beef or pork
    • Preservatives
    • Dyes

Patients should be particularly aware of allergic reactions that occur at the site where insulin is injected.

  • Thyroid, liver or kidney disease.
  • Severe infection.
  • Lung conditions. People who have asthma, bronchitis or emphysema should not use the recently approved inhaled insulin, according to the U.S. Food and Drug Administration (FDA).

Patients should inform their physician of other medications or vitamins they are taking. These can skew the results of testing for glucose (blood sugar) or ketones (potentially dangerous chemicals produced when the body uses stored fat instead of glucose for energy). Also, consult a physician before taking over-the-counter cold or allergy medications, as these may contain alcohol or sugar.

Patients having surgery, including dental surgery, should tell their physician or dentist that they are taking insulin. Surgery can substantially increase or decrease the effect of insulin.

Smokers should inform their physician upon quitting smoking. Insulin dosages for such patients may need to be adjusted.

Other medical conditions that can affect insulin dosages include:

  • Changes in female hormones for women (during puberty, pregnancy or menstruation)
  • High fever
  • Severe infection
  • Emotional stress
  • Overactive or underactive adrenal glands
  • Underactive pituitary gland
  • Diarrhea
  • Gastroparesis (slow stomach emptying)
  • Intestinal blockage
  • Vomiting

Potential side effects of insulin

Side effects are not common with insulin, but they can occur. The most common side effects are related to low blood glucose (hypoglycemia) or high blood glucose (hyperglycemia).

Low blood glucose, or low blood sugar, can be caused by the interaction of the following factors:

  • Food
  • Exercise
  • Medication

Too much or too little of any of these three factors can affect glucose levels. Low blood glucose can be triggered by:

  • Not enough food.
  • Exercising more than usual without taking an adequate meal/snack to cover that exercise.
  • Drinking significant amounts of alcohol.
  • Taking certain medicines.
  • Using too much insulin or the wrong type.
  • Sickness, particularly vomiting or diarrhea.

Symptoms differ from person to person but include:

  • Shakiness
  • Dizziness
  • Rapid heartbeat
  • Sweating
  • Confusion
  • Blurred vision
  • Headache
  • Numbness or tingling of the mouth
  • Fatigueor weakness
  • Paleness
  • Hunger (polyphagia)

Diabetic individuals should have a physician-directed plan for treating hypoglycemia. In general, people who experience symptoms of hypoglycemia should immediately consume a food or beverage with quick-acting sugar and call their physician. Foods with quick-acting sugars include:

  • Hard candy
  • Fruit juice or nondiet soft drink (4 to 6 ounces)
  • Glucose tablets or gel
  • Corn syrup or honey (1 tablespoon)
  • Sugar cubes (six half-inch cubes) or table sugar dissolved in water

Chocolate contains fat that slows sugar from entering the bloodstream.

High blood glucose is a serious problem that can lead to a bloodstream disorder called diabetic ketoacidosis. This involves a potentially life-threatening buildup of acids called ketones and can trigger a diabetic coma. The following conditions can trigger hyperglycemia:

  • Diarrhea, fever or infection
  • Lack of insulin or forgetting to take insulin medication
  • Failure to exercise
  • Overeating or not complying with the restrictions of a meal plan

Symptoms associated with high blood glucose tend to appear more slowly than those of low blood glucose. Diabetic patients should seek immediate medical care if they experience the following symptoms:

  • Excessive thirst (polydipsia)
  • Dry mouth
  • Tiredness
  • Fruit-like breath odor
  • Flushing
  • Dry skin
  • Frequent urination (polyuria) and increased volume
  • Unexplained loss of appetite, nausea or vomiting
  • Breathing difficulties

A physician also should be informed if the following symptoms appear and are severe or persistent:

  • Redness, swelling and itching at the injection site
  • Changes in the skin, including swelling, depression at injection site or thickening
  • Fat buildup or fat breakdown at the injection site

Immediate medical consultation is also necessary with the following symptoms:

  • Seizures
  • Loss of consciousness
  • Severe sunburn
  • Yellowing of the skin or eyes (jaundice)
  • Light-colored stools
  • Dark urine
  • Unusual bleeding or bruising
  • Fever
  • Sore throat
  • High levels of ketones in the urine (ketosis, a possible sign of ketoacidosis)

Some patients who use insulin may experience weight gain. This can cause additional health problems, particularly in patients with type 2 diabetes.  However, one recent two-year study of low-dose insulin prescribed to children at risk of type 1 diabetes found that the treatment caused no change in weight.

Other potential, but rare, side effects include loss of fatty tissue (lipodystrophy) at the site where the insulin is injected, and allergic reactions such as swelling (edema).

Drug or other interactions with insulin

Patients should consult their physicians before taking any additional prescriptions, over-the-counter medications, nutritional supplements or herbal medications. Of particular concern to individuals taking insulin are:

  • Alcohol. Patients should ask their physician for guidelines about how much is safe to drink, as alcohol increases glucose (blood sugar). Usually, smaller amounts of alcohol do not cause problems. But larger amounts taken in a single sitting or over long periods of time (especially without food) can enhance insulin’s effect in lowering glucose levels and keeping them low for longer periods of time.
  • Beta blockers. These common blood pressure drugs (antihypertensives) can increase the likelihood of both high glucose levels (hyperglycemia) and low glucose levels (hypoglycemia). In addition, these medications sometimes mask symptoms of hypoglycemia, such as rapid heartbeat. They can also keep glucose levels low for longer periods of time. Other antihypertensives that may interact with insulin include ACE inhibitors and diuretics.
  • Corticosteroids. These drugs often increase glucose levels. As a result, insulin dosages may have to be increased during corticosteroid treatments and for a while after treatment is completed.
  • Pentamidine. This antiprotozoal pneumonia drug may cause the pancreas to release insulin too quickly. This causes blood glucose to first drop too low, then to become too high.
  • Aspirin. Using aspirin can increase the action of insulin and may cause hypoglycemia.
  • Oral contraceptives. Birth control pills may decrease the effect of insulin.

Other medications that may interact with insulin include some antidiabetic agents, diet pills, MAO inhibitors (class of antidepressants), niacin (a cholesterol-reducing drug), sulfa antibiotics, thyroid drugs, tranquilizers and anabolic steroids.

In addition, people who smoke or have quit smoking within the past six months should not use inhaled insulin, according to the U.S. Food and Drug Administration (FDA).

Symptoms of insulin overdose

Symptoms of overdose can be similar to the medication’s side effects but are usually more severe. The most common symptoms of insulin overdose is hypoglycemia (low blood glucose).

Pregnancy use issues with insulin

Consulting the physician is vital for pregnant women. Insulin needs change for women during and after pregnancy. Women must control glucose (blood sugar) during pregnancy to prevent excessive weight gain of the fetus (macrosomia) or high or low glucose in the baby.

Insulin does not pass into breast milk. However, nursing mothers generally need less insulin. Regular glucose testing can help to detect the need to change insulin dosages. When insulin is used during breastfeeding, the glucose levels in the mother and the baby should be monitored.

Child use issues with insulin

Children require more insulin as they grow. Children’s bodies are especially sensitive to the effects of insulin, making low glucose levels (hypoglycemia) more likely. This is a condition that often lasts until puberty. Children may require higher insulin use before puberty and maybe be able to lower the usage after puberty.

Elderly use issues with insulin

Insulin’s effect on elderly patients is similar to that of the medication’s effect on other adults. However, initial symptoms of low or high glucose (blood sugar) are not always as evident in older adults. This means it is easier for older adults to slip into hypoglycemia, or a state of low blood sugar.

In addition, older adults may have vision impairments or other health problems that make it more difficult for them to measure or inject medicine. Special training and equipment are available for patients in this age group.

Ongoing research

Researchers have been working on several forms of insulin medications that a patient can inhale. The U.S. Food and Drug Administration (FDA) described the first inhaled insulin, approved in 2006, as the biggest advance in insulin therapy since discovery of the hormone in the 1920s.

Other insulin research includes work on potential treatments such as:

  • Development of an insulin patch placed on the skin to provide a continuous dose
  • Insulin pills, drops and other forms of orally administered insulin that can take effect instead of being digested
  • Development of drugs to delay the decline of insulin production in type 1 diabetes
  • Dietary supplements made of resistant starch that may improve the body’s response to insulin and help avoid insulin resistance

Other developments in insulin research include the recent discoveries that:

  • Insulin, normally made in the pancreas, can also be manufactured by abnormal cells in other parts of the body. Malfunctioning bone marrow cells that create insulin have recently been linked to the nerve disease diabetic neuropathy.
  • Creation of beta cells, the insulin-making cells of the pancreas, may still occur even in people with long-standing type 1 diabetes. Such findings could lead to a treatment for type 1 diabetes.
  • Insulin is needed for the formation of blood vessels. Researchers at Joslin Diabetes Center say the discovery could improve treatment of diabetic heart conditions.
  • Low-dose intravenous insulin supplied shortly after a heart attack may curb inflammation and cardiac damage.
  • People with migraine headaches appear to have reduced sensitivity to insulin. The discovery may account for the previously noted link between migraines and vascular diseases and lead to treatments that ease migraines by improving sensitivity to insulin.
  • Lack of insulin in the brains of lab rodents appears to cause an Alzheimer’s-like dementia. Another study found that nasally delivered insulin seemed to improve memory in people with mild cognitive impairment or Alzheimer’s disease. Some researchers are exploring whether Alzheimer’s disease might be a form of diabetes involving insulin resistance in the brain. In addition, recent research has found that insulin receptors in brain, as in other body tissues, play an important role in controlling glucose (blood sugar).

Questions for your doctor about insulin

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about insulin:

  1. What roles do insulin and insulin resistance play in my diabetes and my risk of diabetic complications? How do insulin and glucose interact?
  2. Do you recommend insulin for me?
  3. Are there any other medications or treatments I can use instead of insulin?
  4. Can weight loss or improvements in exercise, diet or other habits reduce my need for insulin? If so, how will this be monitored?
  5. Which type or types of insulin do you recommend for me?
  6. How often do I need to take insulin, and when? What is my dosage?
  7. How often will I need to check my glucose?
  8. Of the various methods of taking insulin, how do their advantages and disadvantages apply to me?
  9. Which method or methods of insulin administration do you recommend for me?
  10. Do you recommend any particular type or brand of this device for me?
  11. Will I need training from a certified diabetes educator in using insulin?
  12. What insertion sites should I use, and how often should I rotate them?
  13. What plan should I have in place in case I have an insulin or glucose emergency?
  14. What are my options if I have trouble with this way of taking insulin?
  15. Can insulin interact with my other medications or cause me any other problems?
  16. Is the new inhaled insulin an option for me?
  17. Are there any other insulin advances nearing approval that could benefit me?
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