Chemotherapy Hypersensitivity Reactions: Signs, Timing, and What to Do

What Exactly Is a Chemotherapy Hypersensitivity Reaction?

When you get chemotherapy, your body is fighting cancer. But sometimes, it fights the medicine too. A chemotherapy hypersensitivity reaction is when your immune system overreacts to a chemo drug, treating it like an invader. This isn’t just nausea or fatigue-it’s an allergic response that can start with a tickle in your throat and turn into trouble breathing in minutes.

It’s not rare. About 5% of people on chemotherapy have some kind of allergic reaction. For some drugs, like carboplatin or paclitaxel, the risk grows the more you get treated. After six doses, your chance of a reaction jumps from less than 1% to over 6%. By the eighth or ninth infusion, it’s nearly one in three. That’s why nurses watch you closely-not just on day one, but every single time you come in.

How Do You Know It’s a Reaction? The Signs by Body System

These reactions don’t look the same for everyone. Some people get a rash. Others feel like they’re going to pass out. The symptoms depend on which part of your body is reacting-and how fast.

• Head and neck: Itchy eyes, swollen lips or tongue, nasal congestion, a strange metallic taste in your mouth.
• Respiratory: Wheezing, chest tightness, coughing, shortness of breath. This is the most dangerous sign. If you can’t catch your breath, it’s time to act.
• Cardiovascular: Dizziness, fainting, rapid heartbeat, low blood pressure. Systolic pressure dropping below 90 mmHg is a red flag.
• Gastrointestinal: Nausea, vomiting, cramps, diarrhea. These can be mistaken for chemo side effects, but they often come with other symptoms.
• Skin: Flushing (sudden redness), hives, itching. About 72% of people report itching. It’s the most common sign.
• Neurological: Anxiety, a feeling that something terrible is about to happen. This isn’t just fear-it’s your body’s alarm system.
• General: Chills, fever, sweating. These come from cytokines, not infection.

Some reactions start mild-just a little redness on your chest-and get worse fast. Others hit hard right away. Either way, if you feel anything unusual during or after your infusion, speak up. Even if you’re not sure.

When Do Reactions Happen? Timing Matters

Most reactions happen during the infusion or within the first hour after. But some show up hours later-or even the next day. That’s why you’re asked to stay in the clinic for at least 30 minutes after your treatment ends.

For most chemo drugs, the first reaction happens early-often after the first or second cycle. But carboplatin is different. It builds up in your system. You might get through five cycles fine, then suddenly react on the sixth. That’s because your immune system is learning to recognize the drug as a threat over time.

Platinum drugs (carboplatin, cisplatin, oxaliplatin) and taxanes (paclitaxel, docetaxel) are the biggest culprits. But monoclonal antibodies like trastuzumab and rituximab can trigger reactions too. Even if you’ve had no problems before, don’t assume you’re safe next time.

What Do Doctors Look For to Diagnose It?

There’s no single blood test that says, “Yes, this is a chemo allergy.” Diagnosis is based on what you feel, what the nurse sees, and when it happens.

Doctors use the same rules for anaphylaxis that apply to bee stings or peanuts: sudden onset, involving skin or respiratory or cardiovascular systems. If you have low blood pressure and wheezing? That’s anaphylaxis. If you have hives and mild nausea? That’s a moderate reaction.

Lab tests can help confirm it. High tryptase levels in your blood (above 11.4 ng/mL) suggest mast cells released histamine. Eosinophils in your blood may rise. Specialized tests like basophil activation can show if your immune cells are reacting to the drug. But these aren’t done right away-they’re for planning future treatments.

Doctors also rule out other things. Is it sepsis? Asthma flare-up? A reaction to contrast dye you got earlier? The symptoms overlap. That’s why timing and context matter.

What Happens If You Have a Reaction? The Protocol

Every oncology unit should have an emergency plan. Here’s what it looks like, step by step.

Mild Reaction (Grade 1):

• Flushing, mild rash, localized itching
• Stop the infusion
• Give diphenhydramine (Benadryl) 25-50 mg IV
• Give dexamethasone 10-20 mg IV
• Monitor vitals for 30 minutes
• If symptoms clear, restart infusion slowly

Moderate Reaction (Grade 2):

• Widespread hives, facial swelling, mild shortness of breath
• Stop infusion immediately
• Give antihistamines and steroids
• Start oxygen if needed
• Wait until symptoms fully resolve
• Restart at half the original rate, over a longer time

Severe Reaction (Grade 3-4) or Anaphylaxis:

• Low blood pressure, wheezing, swelling of throat, chest pain, fainting
• STOP THE INFUSION. NOW.
• Give epinephrine 0.3-0.5 mg intramuscularly (thigh)
• Call for help-this is a code situation
• Start IV fluids
• Give oxygen
• Position you flat with legs raised
• Repeat epinephrine every 5-15 minutes if needed
• Intubation may be needed if airway is swelling

Epinephrine is the only thing that stops anaphylaxis. Antihistamines and steroids help with symptoms, but they don’t save your life. That’s why every chemo room must have epinephrine on hand-not in a cabinet, but within reach.

Can You Still Get Chemo After a Reaction?

Yes-but only with a plan. If you had a mild reaction and need to keep using the same drug, your team may do a desensitization protocol.

This means giving you the drug in tiny, slowly increasing doses over 4 to 12 hours. You’re watched constantly. Nurses check your breathing, blood pressure, and heart rate every 5-15 minutes. You’re given antihistamines and steroids before and during.

It’s not risk-free. But for many people, it’s the only way to keep fighting cancer. If you had a severe reaction-especially with breathing or blood pressure trouble-your team will likely switch you to a different drug. Some drugs, like liposomal doxorubicin or L-asparaginase, are more likely to cause reactions. If you reacted to one, they’ll avoid similar ones.

How to Prevent Reactions Before They Start

Prevention isn’t luck-it’s science. For high-risk drugs, premedication is standard.

For taxanes (paclitaxel, docetaxel), you’ll usually get:

• Dexamethasone 20 mg IV-12 hours and again 6 hours before
• Diphenhydramine 50 mg IV-30 minutes before
• Famotidine 20 mg IV-30 minutes before

This combo cuts reaction rates by more than half. For carboplatin, some centers give antihistamines and steroids too, even if you’ve never reacted before. It’s a gamble-but the risk of a reaction later is high.

Infusion speed matters. Slowing down the drip can prevent reactions. If you had a mild reaction before, they’ll give the next dose over 90 minutes instead of 30.

What You Can Do: Patient Responsibilities

You’re not just a patient. You’re part of the team.

• Always tell your nurse if you’ve had a reaction before-even if it was years ago.
• Report anything unusual: a funny taste, a tingling hand, a sudden chill. Don’t wait.
• Know your drug names. Write them down. Ask what you’re getting each time.
• Don’t assume you’re safe just because you’ve had five good infusions. Carboplatin doesn’t care.
• If you feel symptoms after leaving the clinic, go to the ER. Don’t wait.

Most people who survive a severe reaction say the same thing: “I didn’t want to bother anyone.” Don’t make that mistake. Your voice can save your life.

What Happens After a Reaction?

Your oncology team will document everything. They’ll update your chart, flag your file, and plan for next time.

You may be referred to an allergy specialist for testing. Some centers do skin tests or blood tests to confirm which drug caused it. That helps avoid it in the future.

If you need to switch drugs, your team will choose an alternative with a lower risk. For example, if you reacted to paclitaxel, they might use nab-paclitaxel or docetaxel instead. Not all taxanes are the same.

And if you’re told you can’t have that drug again? That’s not a failure. It’s protection.