The only reliable way is through the checks your doctor does: asking how you feel, examining you, blood tests and scans such as CT, MRI or PET [1]. How severe the side effects are does not show how well chemotherapy is working: strong side effects do not mean a stronger effect, and having no side effects does not mean the treatment is not working [1]. If the tumour has shrunk or disappeared on scans, the treatment has worked; a tumour that stays the same can also be a good result, because one of the aims of chemotherapy is to stop or slow the growth of the cancer [1, 11].
Key points
- Whether chemotherapy is working is judged by checks: asking how you feel, examining you, blood tests and scans such as CT, MRI or PET [1].
- Side effects are not a sign that chemotherapy is working. Severe side effects do not mean a stronger effect, and no side effects do not mean the treatment is not working [1].
- A tumour that stays the same can also be good news, because one of the aims of chemotherapy is to stop or slow the growth of the cancer [1, 11].
- A single rise in a tumour marker in the first weeks of treatment is not always bad; it is sometimes temporary and is seen in people whom the treatment is helping [7, 8].
- With immunotherapy, a tumour sometimes looks bigger at first and then shrinks. This is uncommon, and your doctor may repeat the scan a few weeks later [5, 6].
- If chemotherapy is not working, there are other options: another drug, taking part in a clinical trial and palliative care to ease symptoms [9].
How to tell if chemo is working: the checks
During treatment you see your doctor regularly. They ask how you feel, examine you and arrange tests and scans [1]. None of these is enough on its own; your doctor puts all the results together.
| Check | What it shows | Note |
|---|---|---|
| Scans (CT, MRI, PET) | Compares the size of the tumours with the images taken before treatment [1, 3] | In studies testing new cancer drugs, a scan every 6 to 8 weeks is considered reasonable; your doctor sets your own schedule [4] |
| Tumour markers (blood tests) | The amount of a substance that some cancers release into the blood [7] | Not enough on its own; it is interpreted together with the other checks [7] |
| Examination and how you feel | Changes in lumps or lymph nodes that can be felt, and changes in symptoms [1, 2] | Always mention new or worsening symptoms |
| PET scan in lymphoma | In many lymphomas, it shows whether cancer cells are still active [10] | The result is reported on a 5-point scale [10] |
Are side effects a sign chemo is working?
No. You cannot tell whether chemotherapy is working from its side effects. Some people think that severe side effects mean the drug is working well and that no side effects mean it is not working; but side effects have nothing to do with how well chemotherapy is fighting the cancer [1].
If you feel well and have few side effects, do not worry. To manage side effects, read: What are the side effects of chemotherapy?
What do scan results mean? Complete response, partial response, stable disease and progression
| Term | What it means | What it means for you |
|---|---|---|
| Complete response | All signs of cancer have gone on the checks; this does not always mean the cancer has been cured [11] | The treatment has worked; your doctor will explain whether treatment continues or stops, and the follow-up plan |
| Partial response | The tumour has shrunk; in technical terms, by at least about a third [3, 5, 11] | The treatment has worked |
| Stable disease | The cancer has neither shrunk nor grown [11]; that is, it has not shrunk enough to count as a partial response or grown enough to count as progression [3] | One of the aims of chemotherapy is to stop or slow the growth of the cancer, so this can also be a good result [1] |
| Progression | The cancer has grown, spread or got worse [11]; in technical terms, it has grown by at least a fifth or a new area of cancer has appeared [3] | Your doctor will talk to you about changing treatment [9] |
If you do not understand a term in your scan report, ask your doctor to explain it in simple words. To read about complete response and what "remission" means: What does remission mean?
Tumour markers going up during chemo: does it mean it is not working?
Not necessarily. In the first weeks of a new treatment, a tumour marker sometimes rises for a while; this is why the American Society of Clinical Oncology guideline says that a rise in breast cancer markers in the first 4 to 6 weeks of a new treatment should be interpreted with caution [7].
In a study of people with bowel cancer that had spread, in about 1 in 9 patients the CEA marker rose after chemotherapy started and then came down. In all of them the rise lasted less than 4 months, and it was seen in people whom the treatment was helping [8].
So your doctor does not look at a single number: they look at the trend of the marker over several tests, together with scans and how you are [7]. Read more: What does a high CEA mean? and What does a high CA-125 mean?
Signs chemo is not working
These signs may mean that the treatment has not worked as hoped. But only your doctor, by putting all the checks together, can say so:
- Scans: the tumour has grown or a new area of cancer has appeared [3, 11].
- Tumour markers: they keep rising over several tests and over a long time, not just once in the first weeks [7, 8].
- How you feel: symptoms caused by the cancer have got worse or new symptoms have appeared; your doctor takes this into account when deciding whether to continue treatment [5].
Why does a treatment that was working stop working? Sometimes cancer cells do not respond to a drug from the start, and sometimes they become resistant to it during or after treatment. This resistance can mean the treatment stops working or the cancer comes back [11]. It is not your fault.
What happens if chemo is not working?
Chemotherapy that has not worked is not the end of the road. It may be time for new decisions about your care [9]. Depending on the type of cancer and how you are, these options may be discussed [9]:
- Another treatment: if one exists for your cancer and its benefit is worth the burden for you.
- Taking part in a clinical trial testing new drugs.
- Palliative care: controlling pain and other symptoms. This care is best started at diagnosis and continued alongside treatment, not only when no other treatment is left [9].
These questions can help you make a better decision: "What is the best I can hope for by trying the next treatment?" and "Is this treatment meant to ease symptoms or to slow the cancer?" [9]. Read more: Stage 4 cancer: what it means and treatment, What is palliative care? and Chemotherapy or targeted therapy?
Signs immunotherapy is working, and how long it takes to work
Whether immunotherapy is working is checked in the same way as chemotherapy: examinations, asking how you feel, blood tests and scans [2]. But immunotherapy sometimes follows a different pattern: the tumour first looks bigger, or a new spot appears, and then it shrinks [5]. This is called "pseudoprogression".
It is uncommon: in studies it is usually seen in 1 in 10 people or fewer, mostly in melanoma [5]. In a large melanoma study, about 1 in 20 patients had a bigger tumour at the week-12 check, but the next check showed that the disease had not really progressed; in a few others this pattern was seen later [6].
So if you are well, your doctor may continue treatment even though a scan shows the tumour has grown, and repeat the scan 4 to 8 weeks later [5]. Judging whether immunotherapy is working can therefore take several months and several scans [5, 6]. Read more: What is cancer immunotherapy? and Side effects of immunotherapy.
Cancer coming back after chemotherapy
Sometimes a cancer that responded well to chemotherapy comes back later. One reason is that some cancer cells are, or become, resistant to the drug [11]. This is why regular follow-up with your doctor continues after treatment ends. If the cancer comes back, treatment options are looked at again [9].
Read more: What is cancer recurrence and how is it followed up? and Fear of the cancer coming back and "scanxiety".
When should I call my doctor?
Do not wait for your next scan; tell your treatment team sooner if:
- symptoms you had from the cancer, such as pain, shortness of breath or weight loss, have come back or got worse [5]
- you have a new symptom that has lasted several days, such as new pain, a new lump or a cough that will not go away
- you are getting weaker day by day and everyday tasks are becoming harder [5]
A temperature of 38 °C or higher, sudden shortness of breath, chest pain, confusion or sudden weakness of an arm or leg need urgent care: call your local emergency number. Warning signs during chemotherapy: Fever and warning signs during chemotherapy.
Frequently asked questions
I feel better: does that mean chemotherapy is working?
Improvement in symptoms caused by the cancer, such as pain, is good news and you should tell your doctor. But only examinations, tests and scans can confirm it [1].
I have very few side effects: does that mean the drug is not working?
No. Side effects have nothing to do with how well chemotherapy is working against the cancer [1]. Having few side effects is good news for your quality of life.
After how many chemo sessions will I know if it is working?
Scans are usually repeated after a few cycles. In studies testing new cancer drugs, an interval of 6 to 8 weeks is considered reasonable [4]; your doctor decides the exact timing according to the type of cancer and treatment. Read more: How many chemotherapy sessions will I need?
The tumour has not shrunk, but it has not grown either: is that good or bad?
My scan shows the tumour has grown, but my doctor is continuing immunotherapy: why?
With immunotherapy, a tumour sometimes looks bigger at first and then shrinks. If you are well, the guidelines allow treatment to continue and the scan to be repeated 4 to 8 weeks later, to find out whether the disease has really progressed [5].
Chemotherapy has not worked: does that mean there is no other treatment?
No. Another drug, taking part in a clinical trial, or palliative care to control symptoms may be options [9]. Talk to your doctor about the aim and benefit of each.
Is a PET scan better than a CT scan?
It depends on the type of cancer. In most solid tumours, response to treatment is measured by the size of the tumour on scans such as CT, and PET is an extra aid [3]. In many lymphomas, a PET scan is the main way of measuring response [10]. Read more: What is a PET scan?
Related pages
Response criteria and interpreting tumour markers (for doctors)
For physicians
- RECIST 1.1: at most 5 target lesions (2 per organ); lymph nodes with a short axis of 15 mm or more are measurable and those under 10 mm are normal. PR: a decrease of at least 30% in the sum of diameters from baseline; PD: an increase of at least 20% from the smallest sum (nadir) together with an absolute increase of at least 5 mm, or a new lesion; SD: neither PR nor PD. FDG-PET is only an adjunct for determining progression [3, 5]. In phase II trials, re-evaluation every 6–8 weeks (timed to coincide with the end of a cycle) is considered reasonable [4]. Calculator: RECIST response; lesson: RECIST, iRECIST and Lugano.
- iRECIST: first progression is iUPD; a clinically stable patient (no decline in performance status, no worsening of disease-related symptoms and no increased need for palliative intervention) continues treatment, with the next assessment at least 4 and no more than 8 weeks later; a further increase of at least 5 mm (or progression in another lesion category) confirms iCPD. Pseudoprogression is typically reported in 10% or less, mainly in melanoma [5].
- KEYNOTE-001 (melanoma, pembrolizumab): of 327 patients with at least 28 weeks of imaging, 24 (7%) had atypical responses: 15 (5%) early pseudoprogression (a ≥25% increase in tumour burden at week 12 not confirmed at the next assessment) and 9 (3%) delayed. Of 592 patients alive at week 12, 84 (14%) had progression by RECIST but not by irRC; 2-year overall survival was 77.6% (no progression by either criterion), 37.5% (progression by RECIST only) and 17.3% (progression by both) [6].
- Markers: in metastatic breast cancer, CEA, CA 15-3 and CA 27-29 are adjuncts only, not to be used alone; interpret a rise in the first 4 to 6 weeks of a new therapy with caution (spurious early rise) [7]. In metastatic colorectal cancer (89 patients), a CEA surge (a rise of more than 20% from baseline followed by a fall of more than 20% below baseline) occurred in 10 patients, lasted less than 4 months in all and was associated with clinical benefit; a CEA rise after starting chemotherapy, unless it lasts more than 4 months, is not an indicator of progression [8].
- Lymphoma (Lugano): response in FDG-avid lymphomas is assessed with PET-CT and the 5-point scale [10].
- Decisions after progression: discuss the aim of the next treatment (symptom control or slowing progression), clinical trials and palliative care, which begins at diagnosis [9]; Early palliative care.
References
11 referencesShow all references, 1 with a study cardClose the reference list
Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
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This page is based on reputable scientific sources and was last updated on 28 October 2026. The content of this website is for educational and informational purposes only and does not replace consultation, diagnosis or treatment by a specialist physician. Consult your treating physician about any treatment decision.
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