An allergy occurs when the immune system identifies a harmless substance as being foreign and produces antibodies to fight the substance. Any substance that causes this reaction is called an allergen.

In some cases, an allergic person can avoid contact with the allergen. However, when people become allergic to medications necessary for their health, avoidance may not be possible. Desensitization (immunotherapy) can help many patients to reduce or eliminate allergy symptoms.

Desensitization is the process of decreasing a patient’s sensitivity to an allergen. This is accomplished over time by injecting the patient with gradually increasing amounts of the allergen. When this process is targeted toward enabling patients to tolerate medications to which they are allergic, it is called drug desensitization therapy.

Not all reactions to medications are allergic. Some are simply side effects, which may be treated by adjusting the dosage of medications or by addressing uncomfortable or inconvenient symptoms. Desensitization will have no effect in these cases, and it is important to determine that a patient is in fact allergic to the medication before beginning drug desensitization therapy.

Drug desensitization is considered only when there is no alternative medication or therapy available to treat a condition. For example, a person with diabetes who becomes allergic to insulin or someone who develops an infection that is resistant to alternative antibiotics might qualify for desensitization.

The patient may continue to have allergic reactions to the drug while desensitization takes place. Antihistamines, corticosteroids, epinephrine or other medications may be given to the patient to suppress any allergic symptoms while the desensitization process is completed. 

Desensitization requires consultation with an allergy specialist (allergist/immunologist) to minimize any risks. It is also necessary to consider the specific allergies, circumstances and physiology of each individual patient. It is important for patients to discuss the particulars of their situation with the physician and determine if desensitization is appropriate for them.

Drug desensitization therapy is a procedure used to enable patients to take medications to which they are allergic. The patient is exposed to gradually increasing amounts of the known allergen over a period of time to decrease the patient’s sensitivity to the drug (desensitization). This decreases the patient’s risk (or the severity) of an allergic reaction to the medication.

Drug allergies are rare, occurring in roughly 1 to 3 percent of people, according to the American Academy of Allergy, Asthma and Immunology. Other causes of adverse or unexpected reactions are more common, and include:

• Side effects. Unwanted effects caused by the normal actions of the drug, such as sleeplessness from decongestants.
  
  
• Toxic reactions. Reactions resulting from overdose or an increased sensitivity to usual doses.
  
  
• Idiosyncratic reactions. Unexpected reactions to medication that do not appear to be related to the immune system.
  
  
• Drug interactions. It is important to limit the interaction between medications and other non-medical stimulants or depressants (such as caffeine and alcohol). These interactions may cause physical symptoms of anxiety and nervousness, heart palpitations, or in the case of alcohol, slow reaction times. Antihistamines interact with alcohol and tranquilizers to depress the central nervous system. This interaction can cause severe drowsiness.
  
  Antihistamines may also react with anti-depressant medications, blood pressure drugs and heart disease or thyroid disease medications. Medications used to treat other conditions such as acid reflux can also cause interactions. Some antacids affect the body's ability to metabolize drugs and other chemicals.

In an allergic reaction, the body mistakes a harmless substance, in this case a medication, for a foreign invader. It launches an attack that leads to allergy symptoms. Allergic reactions usually do not occur with the first exposure to the drug because it takes time for the immune system to produce antibodies to react to the medication. This process is called sensitization.

Once the body is sensitized to the drug, the patient will experience allergy symptoms whenever the drug is encountered. Common medication allergy symptoms include hives (urticaria) and swelling beneath the skin (angioedema). In rare circumstances a drug will provoke anaphylaxis, an allergic reaction involving two or more body systems. Anaphylaxis can lead to the potentially fatal anaphylactic shock if not treated immediately.

When a patient is allergic to a specific medication that offers the most effective treatment for a condition, desensitization may enable the patient to benefit from the treatment without severe allergic reactions. Desensitization therapy is possible for some medications, but not for others.

For example, antibiotic and insulin allergies have been successfully treated under controlled circumstances in some patients through drug desensitization therapy. Any patient who cannot be treated with epinephrine (adrenaline that is used to reverse symptoms of anaphylaxis) should not consider drug desensitization therapy, unless otherwise directed by a physician.

Physicians usually determine whether a patient is a good candidate for desensitization through determining the:

• Patient’s current state of health
• Specifics of the patient’s allergy history
• Type and severity of allergy symptoms
• Result of allergy tests
• Necessity of the medication to the patient’s immediate health

The state of the patient’s health is always a consideration because ill health can make an allergic reaction more dangerous. Other adverse effects could result because the body is already in a weakened state.

Drug desensitization therapy is usually completed over a much shorter time period than other forms of desensitization. Allergy shots to treat other forms of allergies (e.g., pollen, dust, insect venom,  pet dander) usually take place over months or even years. In some cases, drug desensitization can take place over a matter of hours or days. This shortened time frame may be referred to as rapid protocol desensitization. Patients may be prescribed allergy medications, such as antihistamines, to relieve symptoms during the desensitization procedure.

Drug desensitization does not last and may need to be repeated if the patient requires the medication for long–term disease management. Desensitization will also need to be repeated if use of the drug is intermittent. For instance, a patient who has been desensitized to an antibiotic takes the medication for only as long as it is necessary. Once antibiotic treatment is stopped for more than 24 hours, the patient is once again at risk for an allergic reaction. If a later infection requires treatment with the same antibiotic, the desensitization process will have to be repeated from the beginning.

Because of the risk for anaphylactic shock, drug desensitization therapy should always take place in a clinical or hospital setting with resuscitation equipment and under the supervision of an allergy specialist (allergist/immunologist).  

People may experience allergic reactions to nearly any medication. However, drug desensitization therapy is not appropriate for most drug allergies, because there are many alternatives.

When necessary, allergies to the following medications have been successfully treated through desensitization therapy:

• Antibiotics. Antibiotics are the largest class of drugs that patients are commonly desensitized to. Penicillin allergy is the most common type of drug allergy, and these allergies can occur in many related antibiotics. For most people, a physician can prescribe other non-penicillin antibiotics. In some circumstances, however, penicillin is the best drug to prescribe, and only under these conditions would desensitization be recommended.
  
  
• Aspirin. A common pain reliever and anti-inflammatory drug that is widely used to prevent heart attacks and stroke in people with heart disease. For people with severe heart disease and an aspirin allergy, desensitization may be beneficial. However, not all people with heart disease and aspirin allergy are candidates for desensitization because success depends on the severity of the allergy. Patients who experience breathing problems with aspirin are better candidates for desensitization than patients who get hives.
  

  

• Insulin. A hormone necessary to treat diabetes, a disorder in which blood sugar (glucose) levels are high because the body does not release or use insulin adequately. Insulin medications are produced using porcine (pig), bovine (cow) or human DNA and the base source material can be an allergen. Some patients with allergic reactions to cow and pig insulin may benefit from desensitization therapy.
  
  Insulin desensitization is commonly performed using human DNA insulin with a rapid desensitization protocol (a decreased time frame in which desensitization is achieved in a matter of hours or days). Insulin allergy is uncommon today because allergies to synthetic human insulin are rare and this is the most common type of insulin used to treat diabetes.

Before beginning drug desensitization, a physician will first determine what drug may be causing the allergic reaction. A series of allergy tests will be performed to ensure that the suspected drug is in fact responsible for allergy symptoms. These tests include:

• Allergy skin tests. A small amount of an allergen is introduced to the patient’s skin to determine if the person is allergic to that allergen. The development of redness or swelling in the test area usually indicates that the patient is allergic to the substance.
  

  

• Allergy blood tests. A sample of the patent’s blood is tested for antibodies (substances that indicate an allergic reaction has occurred) to a specific allergen.

Drug desensitization therapy takes place in a hospital or clinical setting for the patient's safety. The patient may be required to spend one day in the hospital, except in the case of aspirin desensitization, which may occur in a physician’s office. Patients should ensure that they have a ride home and expect that they may not feel well for a short time after therapy.

Desensitization for other allergies (e.g., pollen, dust, insect venom or pet dander) usually takes place over months. Drug desensitization therapy, however, typically takes place over hours or days. This decreased time frame may be referred to as rapid protocol desensitization. The type of medication influences the length of time for desensitization. For instance, desensitization for antibiotic allergies typically takes place over a matter of hours. Other desensitization protocols (e.g., aspirin allergies) take longer, occurring over a number of days.

The patient is introduced to small but increasing amounts to the drug either through oral ingestion or injections. The method of drug desensitization is determined by the type of drug, allergic symptoms and other factors. Desensitization may be more effective for some medications when administered through a particular method.

After the first dose is introduced, patients will be asked to wait for a specific time period until the next dose. The patient remains at the facility during the entire desensitization therapy procedure and will be monitored for some time after each dose to ensure that a significant allergic reaction is not occurring. Should allergy symptoms appear during the desensitization therapy, allergy medications should be administered to relieve the symptoms. These commonly include antihistamines, corticosteroids or epinephrine (in the case of anaphylactic shock). Most allergic reactions occurring during drug desensitization therapy can be managed with these allergy drugs without interrupting the desensitization process.

The time between doses will vary from a few minutes or hours, to a few days. Also, the time between doses may increase after each dose until the patient reaches the “maintenance” phase of immunotherapy.

For some medications, such as aspirin, patients will still need periodic doses of the drug to maintain their tolerance to the substance after desensitization is achieved. How long it takes until maintenance is achieved depends on the medication. For instance, antibiotic desensitization is rarely followed by a maintenance stage, because the need for these medications is usually immediate and short-term.

What happens after the drug desensitization therapy depends on the drug allergy, the patient's condition and other factors. Some patients can go home right after the therapy but others will need to stay for some time to be monitored by the physician.

Some patients will continue to take the drug for maintenance after the initial desensitization therapy takes place. Maintenance doses may be achieved in five to 10 days. This method is often used for aspirin allergies. The physician will tell patients what is appropriate for their situation.

Long-term maintenance doses require an administration schedule specific to the patient and the drug. Maintenance doses will change to a daily, weekly, biweekly or monthly schedule depending on the situation.

Desensitization may be repeated in some cases of medicine allergies. When use of a medication is intermittent or interrupted, the desensitization process will have to be repeated from the beginning. For instance, patients desensitized to an antibiotic will stop taking the medication once the infection is resolved. If those patients have a similar infection in the future, they will need to complete desensitization therapy again in order to take the antibiotic. Their desensitization to the drug was lost when they stopped taking the medication.

Patients should ensure that they maintain the best state of health they can after therapy. Adequate rest, good nutrition and exercise are important. This will help boost the immune system and benefit their overall health. Patients should stay in contact with the physician and raise any issues when necessary.

Drug desensitization carries higher risks than immunotherapy. The therapy is usually recommended only when a patient's health requires the use of a specific drug for which there are no adequate substitutes. The main risks of desensitization include:

• Therapy failure. Drug desensitization therapy is not effective with all medication or for all patients. If the therapy does not work, alternative (if less effective) therapies may be needed to control the medical condition.
  
  
• Severe reaction. Whenever allergic individuals come into contact with the allergen to which they are sensitive, a severe allergic reaction is possible. The risk is higher with drug allergies, which is a common trigger of anaphylactic shock. Desensitization should always take place in a clinical or hospital setting under the supervision of an allergy specialist (allergist/immunologist).
  

  

Other considerations are the state of the patient’s health. Exposure to the allergen during desensitization may cause an allergic reaction. If a patient is in poor health, then even minor reactions could pose significant health risks. The risk of causing an allergic reaction may outweigh the potential benefits for patients with heart problems or life-threatening conditions.

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

1. Am I a candidate for drug desensitization therapy?
   
   
2. Are there alternative medications available to me?
   
   
3. How long will my drug desensitization therapy take to complete?
   
   
4. What risks do I face by undergoing this type of therapy?
   
   
5. Can you explain the process to me in detail?
   
   
6. Will the effects of drug desensitization therapy last or will I need to undergo the treatment again in the future?
   
   
7. Will I require hospitalization?
   
   
8. What happens if I develop allergy symptoms during desensitization therapy?
   
   
9. What happens if I go into anaphylactic shock during desensitization therapy?
   
   
10. How will I feel after desensitization therapy?