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).