Medical Oncologist
During treatment
Infusion Reactions: — What Happens and How They Are Managed
An infusion reaction is your body responding to a monoclonal antibody entering the bloodstream. Most reactions are mild, happen during the drip, and are managed by the clinical team on the spot. Knowing what to tell the nurse — and what to watch for after you leave — is what keeps you safe.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- Most happen during the drip — Clinical staff are present for the most likely window, so reactions are usually caught and managed quickly.
- Mild symptoms are common — Flushing, chills, and mild fever are frequently reported and are usually managed by slowing the drip rate.
- Tell the nurse immediately — Do not wait until the infusion ends to mention how you feel. Earlier is always better.
- Delayed reactions are less common but real — Symptoms that appear hours after leaving the clinic need the same prompt attention as those during the drip.
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An infusion reaction is your body responding to a monoclonal antibody entering the bloodstream — usually flushing, chills, mild fever, or itching during the drip. Most reactions are mild and managed by the nursing team by slowing or pausing the infusion. Severe reactions are uncommon but real. Difficulty breathing, throat tightening, chest pain, or fainting at any point is an emergency.
If a symptom below applies to you, do not wait for a callback. Call 1800-202-8726 or go to your nearest emergency department.
Is an infusion reaction dangerous?
Most infusion reactions are mild and manageable. Flushing, chills, mild fever, mild headache, and itching are uncomfortable but not dangerous, and clinical staff are trained to respond to them during the infusion itself.
When you report a symptom or the nurse notices a reaction, they will typically slow or pause the drip and may give you an antihistamine, paracetamol, or a steroid. Many people complete their infusion the same day once symptoms settle.
Severe reactions — where breathing or blood pressure are affected — are uncommon. They are the reason you are observed for a period after the infusion ends before you are allowed to leave. That observation window is not a formality.
How serious is your reaction?
| What you feel | Likely severity | What the team does |
|---|---|---|
| Flushing, mild chills, mild fever, mild headache, mild itching | Mild | Slow the infusion; give paracetamol or antihistamine; watch closely |
| More pronounced chills or rigors, hives (urticaria), mild breathlessness | Moderate | Pause the infusion; give antihistamine and steroid; restart at slower rate when settled |
| Difficulty breathing, chest tightness, swelling of lips or throat, dizziness, blood pressure drop | Severe | Stop infusion immediately; emergency medications given; transfer to emergency care if needed |
| Typically starts | During the infusion or within the first hour or two | Delayed reactions can appear up to 24 hours later — the team will tell you what to watch for at home |
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What should you do if you feel unwell during the infusion?
- Tell the nurse straight away — do not wait until you feel worse or until the drip ends.
- Describe exactly what you feel: where in your body, when it started, and how quickly it came on.
- Do not try to push through it or assume it will pass on its own.
- Stay seated and calm while the team assesses you — anxiety can make symptoms feel more intense.
- Before you leave, ask the team what to watch for at home and when to call.
What is normal after you get home — and what needs a call?
Mild fatigue, a lingering headache, or low-grade muscle aches on the day of the infusion are commonly reported and usually settle by the following morning.
**Call your oncology team the same day if:** flushing, hives, or fever reappear at home; any new rash spreads or is accompanied by itching you cannot settle; or you notice any swelling of the face, lips, or throat.
Do not call first if you have difficulty breathing, chest pain, throat tightening, or feel faint. Go directly to emergency. At home, there is no team present to manage those symptoms.
Did you know?
Pre-medication — typically an antihistamine, paracetamol, and sometimes a steroid — given before your infusion is a standard part of monoclonal antibody protocols recommended by NCCN and ASCO. It meaningfully reduces both the rate and severity of infusion reactions.
If you had a reaction and another infusion is planned, ask your team whether your pre-medication should be adjusted before the next dose.
Source: NCCN Clinical Practice Guidelines in Oncology; ASCO Guidelines for Management of Immune-Related Adverse Events
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Common questions
Frequently asked questions
Will I definitely have an infusion reaction?
No. Reactions affect a proportion of people receiving monoclonal antibodies, and many complete their full course without one. Risk is highest during the first infusion and tends to decrease with each subsequent dose as the body becomes more familiar with the treatment. Pre-medication further reduces the likelihood. Because you cannot predict in advance whether you will react, the clinical team observes you throughout every infusion.
If I had a bad reaction the first time, can I still have the second infusion?
Often yes, with modifications. For mild to moderate reactions, the standard approach is to adjust pre-medication, restart the infusion at a slower rate, and monitor more closely. For a severe reaction, your oncologist will weigh the risk of repeating against the benefit of continuing treatment. A bad first reaction does not automatically end the course — but it does change how every subsequent infusion is managed.
Can I take an antihistamine at home before my infusion to reduce the risk?
Only take anything before your infusion if your oncology team has explicitly told you to. Pre-medication is given under clinical supervision because the type, dose, and timing depend on which monoclonal antibody you are receiving and your individual history. Taking something at home without guidance could interfere with what the team plans to give you, or mask early symptoms during the infusion that the team needs to see.
How long after the infusion can a reaction still appear?
Most reactions happen during the infusion or within the first hour or two after it ends — which is why you are observed before leaving. Delayed reactions can appear up to 24 hours later, though they are less common. Any symptom from the red flag list that appears within 24 hours of an infusion should be treated as potentially related, regardless of how well you felt immediately after.
Is an infusion reaction the same as an allergic reaction?
They overlap but are not identical. Many infusion reactions involve cytokine release — the immune system is activated when the drug binds to cancer cells and releases signalling chemicals that cause symptoms. True allergic reactions involve a different immune mechanism. Both can range from mild to severe, and the outward symptoms can look similar. The distinction matters for how your team adjusts pre-medication and manages future infusions, so it is worth asking what type of reaction yours was.