The short answer
Chemotherapy side effects often follow your treatment cycle. Nausea can start within 24 hours or come 1 to 7 days later. White blood cells are lowest (nadir) about 7 to 10 days after chemo. The rest period lets your body recover.
Side effects occur in cycles that match your chemotherapy schedule.
Nausea can start within 24 hours of treatment (acute) or 1 to 7 days after (delayed).
The American Cancer Society puts the nadir — the lowest blood counts — at about 7 to 10 days after chemo.
When chemo given every 2 to 3 weeks causes hair loss, it usually starts about 2 to 3 weeks after the first cycle.
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This page is for people starting chemotherapy who want to know when side effects tend to appear during each cycle.
The short answer
Chemotherapy side effects usually follow a cycle tied to your infusion schedule. Nausea can start within 24 hours, or come 1 to 7 days later. Fatigue is the most common side effect. Blood cell counts then drop to their lowest point, called the nadir, before they climb back up ahead of your next cycle. The day ranges below are a common pattern, not a rule. Ask your team for the timing that fits your own drugs.
Chemo cycle timeline
[ Day 1: Infusion ] ➔ [ Days 1-7: Nausea may occur ] ➔ [ About Days 7-10: Infection Risk (Nadir) ] ➔ [ Rest period: Recovery ]
Day 1 (Infusion Day)
- What happens: you receive pre-meds (medicines given before chemo to prevent reactions and nausea) and chemotherapy.
- Common feelings: fatigue. Acute nausea and vomiting can happen within 24 hours after treatment starts. Ask your team what each pre-med is for and what side effects it can cause.
Days 1 to 7 (Delayed Nausea)
- What happens: delayed nausea and vomiting can happen 1 to 7 days after treatment.
- Common feelings: nausea and fatigue. Tell your team if your anti-nausea medicines are not working.
About Days 7 to 10 (The Nadir Window)
- What happens: chemotherapy slows the bone marrow for a while. Your blood counts, especially white blood cells, reach their lowest point, the nadir. The American Cancer Society puts the nadir at about 7 to 10 days after chemo; the drugs you are on set the actual timing.
- Key concern: infection risk is highest here. Ask your team what precautions they recommend, such as handwashing and avoiding crowds. Check your temperature if you feel chilled or unwell.
Rest period (Recovery)
- What happens: chemo is often given in cycles, with a rest period after treatment. The rest gives your body a chance to recover and build new healthy cells.
When does hair loss start?
Not every chemotherapy drug causes hair loss. The American Cancer Society says that with chemo given every 2 to 3 weeks, hair loss usually starts about 2 to 3 weeks after the first cycle. Some people have pain, burning, or tingling of the scalp. The National Cancer Institute says hair often grows back in 2 to 3 months after treatment ends. After radiation therapy, regrowth often takes 3 to 6 months.
When to get help sooner
The nadir window is the part of the cycle where a small infection can turn serious fast. Do not wait for your next visit.
- Call your doctor immediately if you have a temperature of 100.4°F (38°C) or higher, or feel chilled and shaky, while on chemotherapy. CDC says a fever during chemotherapy treatment is a medical emergency, and tells you to call straight away at that temperature — at any hour. If you cannot reach anyone and feel unwell, go to an emergency department and tell the staff you are on chemotherapy.
- Call 911 or go to an emergency department if you have trouble breathing, chest pain, confusion, or you feel faint. Do not drive yourself.
- Call your care team right away if you cannot keep fluids down, have not passed urine in many hours, or feel dizzy when you stand.
- Call 911 or go to an emergency department if you have bleeding that will not stop. Do not drive yourself.
- Call your care team right away if you notice new bruises, or blood in your urine or stool.
- Call your care team within a day or two if fatigue, constipation, or mouth soreness is worse than in your last cycle.
Ask your team before taking any fever-reducing medicine, such as acetaminophen. It can mask a fever they need to know about.
Sources
- CDC — Fever and Cancer Treatment
- American Cancer Society — Low White Blood Cell Counts (Neutropenia)
- National Cancer Institute — Chemotherapy to Treat Cancer
- National Cancer Institute — Infection and Neutropenia during Cancer Treatment
- National Cancer Institute — Hair Loss (Alopecia) and Cancer Treatment
- National Cancer Institute — Nausea and Vomiting (PDQ)
- American Cancer Society — Coping With Hair Loss
Common questions
When does nausea from chemo start?
The National Cancer Institute says acute nausea and vomiting happen within 24 hours after treatment starts. Delayed nausea and vomiting happen 1 to 7 days after treatment. Ask your team how to take your anti-nausea medicines.
What does 'nadir' mean in chemotherapy?
Nadir refers to the point in your chemotherapy cycle when your bone marrow's blood cell counts (especially neutrophils, which fight infection) reach their lowest level. The American Cancer Society puts that point at about 7 to 10 days after chemo, though it varies by drug.
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Last updated: 2026-09-29Next planned review: 2027-01-23
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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