Medical Oncologist
Alectinib (Alecensa)
Alectinib Side Effects: — Full List and Timeline
Alectinib (Alecensa) causes a predictable set of side effects. Most are manageable, but a few need your team's attention the same day. Knowing what to expect — and when to call — means you are less likely to be caught off guard.
Medically reviewed by Dr. Bharati Devi Gorantla, Medical Oncologist, MBBS · MD · DM (Adyar, Chennai) · ECMO · MRCP SCE (UK) · Last reviewed August 2026
- Sun protection is not optional — Alectinib makes your skin highly sensitive to UV light. Burns can happen within minutes without SPF 50 or higher sunscreen.
- Blood tests are your early warning system — Raised muscle enzymes and liver enzymes are common side effects that cause no symptoms. That is why the monitoring blood tests matter.
- Fatigue is real — Most people on alectinib feel more tired than usual. It is the side effect people are often least prepared for.
- Breathlessness needs a same-day call — New or worsening breathlessness on alectinib is never ordinary shortness of breath — it needs urgent assessment.
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Alectinib most commonly causes fatigue, constipation, muscle pain, swelling in the legs, and sensitivity to sunlight. Most people notice some of these within the first few weeks. The side effects that need urgent attention are new breathlessness, jaundice, dark urine, or a heart rate that feels very slow or irregular.
What side effects do most people notice on alectinib?
Fatigue and constipation are the two side effects most people experience. Fatigue is often worst in the first few weeks and tends to ease somewhat as your body adjusts. Constipation usually responds to extra fluids, more fruit, and a gentle laxative — ask your team what they recommend before starting anything new.
Muscle aching, ankle and leg swelling, and nausea are also common. Nausea is worst early on and often settles after the first month. Taking alectinib capsules with food rather than on an empty stomach reduces nausea for many people.
Sun sensitivity catches most people off guard. Your skin becomes much more reactive to ultraviolet light on alectinib — a few unprotected minutes outdoors can cause a burn that would normally take an hour. Apply SPF 50 or higher every morning, even on cloudy days and near windows indoors, and cover your skin when you go outside.
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Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
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Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
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Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
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Which side effects need a call, and which can wait?
| Side effect | How common | When it starts | Call your team if |
|---|---|---|---|
| Fatigue | Very common | First few weeks | Suddenly much worse, or you cannot get out of bed |
| Constipation | Very common | First few weeks | No bowel movement for several days, or abdominal pain |
| Muscle pain or aching | Common | Weeks to months in | Severe weakness, difficulty walking, or dark urine alongside it |
| Leg or ankle swelling | Common | First few months | Sudden increase, or swelling with new breathlessness |
| Nausea | Common | First weeks — often settles | Unable to eat or drink for a full day |
| Sun sensitivity | Common | Any time on treatment | Blistering or raw skin after brief exposure |
| Slow heart rate | Common — usually no symptoms | Often early in treatment | Feeling faint, dizzy, or short of breath |
| Raised liver enzymes | Common — blood test finding | Usually first few months | Yellowing of eyes or skin, or very dark urine |
| Raised muscle enzyme (CPK) | Common — blood test finding | Any time | Severe muscle weakness or dark brown urine |
| Breathlessness or new cough | Less common | Any time | Same day — do not wait |
| Typically starts | — | Most side effects appear in the first few months. Pneumonitis and muscle-related changes can occur at any time on treatment. | — |
Did you know?
Sun damage on alectinib can develop within minutes — not the hour or more it might normally take. NCCN guidance for patients on ALK-targeted therapy recommends applying SPF 50 or higher every single day for the full duration of treatment, including cloudy days and days spent mostly indoors.
Many patients discover this the hard way after a short outdoor errand. A brief walk without sunscreen is enough.
Source: NCCN Guidelines for Non-Small Cell Lung Cancer
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Common questions
Frequently asked questions
Is it normal to feel very tired on alectinib?
Fatigue is the most commonly reported side effect of alectinib and affects the majority of people taking it. It is often worst in the first few weeks and tends to improve somewhat with time, though it rarely disappears completely. Rest when you need to and keep to a gentle routine. Tell your team if it is severe enough to stop you doing everyday things, because adjusting other aspects of your care can sometimes help. Sudden severe fatigue — especially alongside breathlessness — needs a same-day call rather than waiting for your next appointment.
What should I do about sun sensitivity on alectinib?
Apply SPF 50 or higher sunscreen every morning, including days you expect to stay indoors, because ultraviolet light through windows can still cause a reaction. Cover your arms and legs when you go outside, and avoid direct sun between 10am and 4pm where you can. Even a short auto-rickshaw ride or a walk to the pharmacy without sunscreen can cause a burn that would not happen normally. This is not optional precaution — it is part of managing the drug safely.
My blood test shows high CPK or liver enzymes — what does that mean?
Raised CPK and mildly raised liver enzymes are common on alectinib and are usually detected on routine blood tests rather than through any symptom you would notice. Your team will monitor the levels at each test and decide whether any change to your treatment is needed. You do not usually need to do anything other than keep your blood test appointments. If you develop severe muscle weakness or pain, or notice yellowing of your eyes or skin, call your team the same day — those symptoms need prompt review rather than waiting for the next scheduled test.
Can I take paracetamol for muscle pain on alectinib?
Ask your oncology team before taking any painkiller, including paracetamol. Alectinib can affect liver function, and some medicines add to that burden in ways that are not always predictable. Your team will advise what is safe for you specifically. Gentle movement and a warm compress often help with muscle aching. If the pain is severe, comes with weakness in your legs, or your urine looks dark, that is a more urgent situation — call your team rather than managing it with painkillers.
Do alectinib side effects get better over time?
Some improve and some stay roughly constant. Nausea tends to ease after the first few weeks. Fatigue often becomes more manageable but rarely resolves fully. Sun sensitivity and the risk of raised blood test values continue for as long as you are taking the drug. Most side effects that are going to appear do so within the first few months, though muscle-related problems and lung inflammation can occur at any point. Tell your team at each appointment how things are — managing side effects well matters for staying on the most effective dose.