For Patients and Families

Mariana C. Castells, MD, Director, Drug Hypersensitivity and Desensitization Center at Brigham and Women's Hospital, describes the process of drug desensitization, a method in which a drug can be safely re-introduced to a patient who has become allergic to their medication. Read the Drug Desensitization video transcript.

What to Expect

What does “rapid desensitization” mean and how it is done?

The definition of an “allergen” is anything to which a person is allergic, such as a medication, a type of food, or an environmental agent such as tree pollen or cat dander. If you develop such an allergy, your body produces an antibody called IgE that works against the “allergen” or allergic agent. Most of the antibodies in your body are called IgG and serve a vital role to protect you against infection. IgE antibodies are different. They work against agents that are not usually infectious. They cause the symptoms of allergies: sneezing and runny nose, red watery eyes, shortness of breath or wheezing, itching, rash, nausea, dizziness, chest tightness, back pain, or loss of consciousness. Allergic reactions can range from the mild (sneezing and runny nose, or a few hives) to the severe (anaphylaxis or cardiopulmonary arrest). “Desensitization” is a method to help a person with an allergy (ies) to tolerate exposure to a specific allergen. You may be familiar with this method, as it is used on an outpatient basis for sensitivity to common allergens such as pollens, dust mites, cat, and dog.

Recently we have developed desensitization techniques for medications, including chemotherapy agents, antibiotics, monoclonal antibodies, aspirin, and others. Desensitization to a certain medication involves giving the medication in slowly increasing amounts, starting with tiny doses, and ending with the full dose prescribed by the physician. The medication is given in the same format (oral, intravenous, or intraperitoneal), but starts more slowly and the process can take much longer.

I had a reaction to my medication, and my doctor wants me to get desensitized – what does this involve?

If you had a reaction to your medication, your immune system may remember it and be primed to have a similar reaction every time you are exposed to that same medication in the future. If your physician believes that this particular medication is the best one for you, and that there is no alternative medication for your treatment, she/he will recommend desensitization. Not all reactions to medications are able to be desensitized. You will need to be evaluated by a Physician who is a member of the Allergy and Immunology at BWH. They will review the nature and severity of your reaction, the potential risks for desensitization, and your need for that particular kind of medication. A risk evaluation process will determine if you can be in the desensitization program, and what your risk is for a reaction during desensitization.

If after careful evaluation you qualify for desensitization, a member of the Allergy team will contact your physician. If your physician agrees to proceed with desensitization, the allergy team will arrange for the time and location for you to undergo your first desensitization.

Depending on the severity of your reaction and your associated medical conditions (cardiac disease, pulmonary disease, etc), you will either be admitted to have your desensitization completed in the Medical Intensive Care Unit (MICU) at BWH or scheduled for an out-patient desensitization that may occur at DFCI or BWH infusion unit. You will be contacted with a tentative date, which should occur on the/or close to the date that your next treatment is due.

If you are required to have your first desensitization in the MICU, the following will occur:
  1. The desensitization will occur at the Brigham and Women's Hospital in the Medical Intensive Care Unit (ICU, or MICU) on the 3rd floor of the Tower Building. It is performed here to make possible very close supervision and to have the expertise of the nurses and staff. Please note that due to the limited availability of beds in the MICU, the date set for your initial desensitization is a tentative target date, not a guaranteed date for admission. A nurse from the MICU, and/or an admitting officer at BWH will be in touch with you regarding date and bed availability. You will receive a call from the desensitization coordinator, Sherwood Alexis, who will let you know that a bed may be available the morning of possible admission to confirm your availability to undergo desensitization. Once a bed becomes available, usually late afternoon, you will be contacted by the desensitization coordinator to inform you of the time and instructions for your arrival. You will be admitted to the MICU the evening prior to your scheduled desensitization so that the desensitization protocol may begin as early as possible the following morning. You may also be contacted by one of the allergists prior to your admission if he or she feels that any of the medications that you are taking need to be held prior to your desensitization (in particular beta-blockers taken for hypertension need to be discontinued at least 24 hours prior to desensitization).
  2. When a MICU bed becomes available, you will come to the BWH admitting office (75 Francis Street, Boston, MA 02115) and someone will bring you to your room in the ICU. Your admission is likely to occur in the PM. The nurse will get you settled, draw some blood for relevant laboratory tests, take your weight, obtain IV access and notify the allergy fellow and your referring physician that you have arrived. If your medication dose is dependent on specific lab values (such as your creatinine, a measure of your kidney function), your doctors may need to wait until these laboratory values are available. In addition, the allergist must wait for your physician to order the dose of your medication. This process usually takes several hours.
  3. When all is prepared, the allergy team will produce a written plan (protocol) for the pharmacy and nurse to follow. An allergy fellow will also come to see you to review your history, examine you, discuss the plan and answer your questions, and obtain your consent for desensitization.
  4. The pharmacist prepares your medication. This step usually takes between 2-3 hours, but may be longer if orders need to be clarified. In the meantime, the doctors and nurses in the ICU will meet with you and order any other medications that you may need during your stay.
  5. When the medication is ready, the nurse will connect you to the appropriate equipment, which usually includes a blood pressure cuff, heart rhythm monitor, and oxygen saturation monitor (also called a pulse oximeter). You will receive pre-medications and then the medication to which you are allergic by a 12-step desensitization protocol. If your initial reaction to this medication was severe, you may receive a longer (16-step) protocol.

Desensitization to an intravenous or intraperitoneal medication involves starting the infusion at miniscule doses, and increasing the rate of administration every 15 minutes, until a “target rate” is achieved. This rate is then maintained until the full dose of medication has been infused. A nurse will be observing you closely during this time. The infusion usually will take between 4 to 8 hours if you have no adverse reactions. (The allergy fellow will review with you the time assigned for your particular desensitization protocol). Of course, it will take longer if the infusion needs to be temporarily stopped due to a reaction, however mild.

Please note: You should expect to stay at least one night in the hospital during your first desensitization. If you arrive in the evening and all goes well, you may be able to return home that following night, but do not expect this. The length of your stay is determined by how long it takes for your infusion to start and finish. This will vary in each case. We will do everything we can to make your time with us a smooth and brief stay.

What if I have a reaction? Will I still get the medication?

If you have a reaction, the nurse will immediately stop the infusion, check your blood pressure and oxygen level, and treat your symptoms with medications, such as Benadryl for hives or swelling, a breathing treatment (albuterol) if you are short of breath or wheezing, or steroids and epinephrine if you have throat tightness or tongue swelling, or have a substantial drop in blood pressure. These symptoms will be treated immediately within the protocol guidelines.

If your symptoms are mild and resolve, the infusion will be restarted at the same rate, and the protocol will continue. If the symptoms are more severe and involve changes in your blood pressure, pulse, or respiration, the ICU doctors and/or the allergist on-call will examine you and decide how best to treat you.

If you do have a reaction, it is usually milder than your original reaction to your medication. Please tell the nurse if you notice any symptoms developing such as rash, hives, swelling, shortness of breath, chest or throat tightness, dizziness, palpitations, chest or back pain.

It is extremely rare that a reaction will be severe enough to prevent completion of your desensitization. It may take longer than originally planned, but the goal will always be to complete the infusion and to provide you with your needed medication.

What to expect when you’re discharged after each desensitization? (Inpatient and Outpatient Settings)

Since you will most likely receive medications that cause drowsiness during your desensitization, for safety reasons, we require you to obtain transportation after you have completed your desensitization. We also encourage you to refrain from driving for the remainder of the day.

Do I need to be desensitized more than once? How long do the effects last?

For drugs that are given on a daily basis, desensitization only needs to be performed once – as long as the medication is being taken daily, the body remembers and keeps a state of desensitization. Those medications include antibiotics used to treat infections, aspirin for prevention of heart attacks, and insulin, used by diabetics.

Although studies have not been performed to test exactly how long a desensitization to medication lasts, it is generally agreed that if more than 2 days have passed since your last dose, the desensitization needs to be performed again. Thus, for patients receiving desensitization for chemotherapy and monoclonal antibodies, every dose of that medication needs to be given via this extended infusion protocol. Your physician will decide how many times, and how frequently, you will receive this particular treatment.

Do I need to be admitted to the hospital each time?

If no reaction or a mild reaction occurs during your initial desensitization, you will receive future desensitizations as an outpatient. If you had a severe reaction during your initial desensitization, you may be admitted to the MICU again. Otherwise, you will be admitted to one of the inpatient care areas. The protocol will be modified to protect you, and to prevent further reactions. The majority of patients only need to come to the ICU once. Once you complete the protocol with minimal or no reactions, you will no longer need to be admitted to the hospital for desensitization.

What happens if all goes well?

For patients receiving chemotherapy by desensitization, if all goes well, additional desensitizations will occur at outpatient facilities. This will be coordinated through your referring physician’s office. Patients getting desensitized to other medications, such as antibiotics, must remember to continue to take them daily, to maintain the state of desensitization. If more than 2 days have passed since your last dose, the desensitization needs to be performed again. Plans for repeat desensitizations (if needed) will be made by your allergist on a case-by-case basis.

Desensitizations given outpatient at the DFCI require early morning arrivals and should be finished by the late afternoon.

  • Again, it is the hospital policy that you will need to arrange transportation after your desensitization.

Our Staff

Desensitization Program Attendings

  • Dr. Marianna Castells
  • Dr. David Sloane
  • Dr. David Hong
  • Dr. Karen Hsu Blatman
  • Dr. Katherine Cahill
  • Dr. Lora Bankova
  • Dr. Laura Fanning

Desensitization Program Fellows

  • Dr. Jared Silver
  • Dr. Kathleen Buchheit

Desensitization Nurse Practitioner

  • Donna-Marie Lynch, FNP-BC

Desensitization Program Coordinator

Sherwood Alexis
Tel: (617) 525-1237

A last note

Please do not hesitate to contact either Dr. Mariana Castells, Dr. David Sloane, Dr. David Hong, Dr. Karen Hsu Blatman, or the allergy fellow on-call, if you have any further questions or concerns, at (617) 732-9850 (BWH Allergy at 850 Boylston Street in Chestnut Hill), or through the BWH page operator at (617) 732-5500 (BWH main telephone number).

Mariana Castells
Associate Professor in Medicine
Harvard Medical School
Director, Adverse Drug Reaction and Desensitization Program
Associate Director, Allergy and Immunology Training Program
Brigham and Women’s Hospital


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