Chemotherapy and radiation therapy cause a wide range of side effects because they damage healthy cells along with cancer cells. Fatigue is the single most common side effect of both treatments, affecting roughly 73% to 85% of chemotherapy patients and nearly everyone receiving radiation. The specific side effects you experience depend on the type of chemotherapy drugs used, the area of your body receiving radiation, and the doses involved.
Why These Treatments Cause Side Effects
Both chemotherapy and radiation work by disrupting cell division, but they aren’t precise enough to spare every healthy cell in the process. Chemotherapy drugs travel through your bloodstream and attack rapidly dividing cells wherever they find them. That means your hair follicles, the lining of your digestive tract, your bone marrow, and your reproductive cells all take collateral damage because they naturally divide quickly.
Radiation works differently but causes similar problems. High-energy particles break chemical bonds inside cells, creating reactive molecules (free radicals) that damage DNA, proteins, and cell membranes. Cells that happen to be in the middle of dividing when radiation hits are especially vulnerable because their DNA is exposed and their repair systems are limited. And because tumors tend to have low oxygen levels, the free radical damage actually occurs more heavily in the surrounding normal tissue than in the tumor itself, which is one reason side effects concentrate near the treatment site.
The Most Common Chemotherapy Side Effects
A prospective study tracking chemotherapy patients over an average of about five and a half months found that fatigue was the most common side effect at 73% to 85% prevalence, followed by diarrhea and constipation (each around 74%). About 63% of patients experienced nausea and vomiting at some point during treatment. Chest pain was the least common, affecting only about 4% of patients. The average patient dealt with side effects roughly 54% of the time they were in treatment.
Beyond the digestive symptoms, chemotherapy commonly causes:
- Hair loss: Many chemotherapy drugs damage hair follicles, causing hair to thin or fall out within weeks of starting treatment. Regrowth typically begins 3 to 6 months after treatment ends, though the new hair often comes in with a slightly different texture or color. That difference is usually temporary.
- Mouth sores and taste changes: The fast-dividing cells lining your mouth are particularly vulnerable, leading to painful sores, dry mouth, or a metallic taste.
- Skin changes: Dryness, rashes, and increased sensitivity to sunlight are common.
Weakened Immune System
Chemotherapy suppresses your bone marrow’s ability to produce white blood cells, the front line of your immune system. After each treatment cycle, your white blood cell count drops to its lowest point, called the nadir. Doctors classify the severity by counting a specific type of white blood cell: below 1,500 cells per cubic millimeter is mild, below 1,000 is moderate, and below 500 is severe. At the severe level, even a minor infection can become life-threatening. A fever during this window is treated as a medical emergency.
To shorten the time you spend at dangerously low counts, doctors sometimes prescribe growth factor injections that stimulate your bone marrow. You’ll still hit that low point, but you’ll recover from it faster, which reduces the window where infections pose the greatest risk.
Nerve Damage From Chemotherapy
Certain chemotherapy drugs damage peripheral nerves, causing numbness, tingling, burning, or shooting pain in your hands and feet. This is called chemotherapy-induced peripheral neuropathy, and it’s remarkably common. In the first month after chemotherapy, about 68% of patients experience it. At three months, the rate drops to around 60%. But at six months and beyond, 30% of patients still have symptoms. For some people, the nerve damage persists for years.
The severity varies depending on which drugs you receive and your cumulative dose. Symptoms often start in the fingertips and toes, then work inward. They can make everyday tasks like buttoning a shirt or feeling the temperature of water genuinely difficult.
Radiation Side Effects by Body Region
Unlike chemotherapy, radiation targets a specific area. Your side effects depend heavily on where the beam is aimed.
- Head and neck: Mouth sores, difficulty swallowing, taste changes, dry mouth, and a less active thyroid gland. Hair loss occurs only in the area being treated.
- Chest: Cough, shortness of breath, difficulty swallowing, and skin irritation over the treatment area.
- Stomach and abdomen: Nausea, vomiting, diarrhea, and bladder irritation.
- Pelvis: Diarrhea, bladder problems, sexual dysfunction, and fertility problems in both men and women.
Fatigue and skin changes at the treatment site are nearly universal regardless of location. The skin in the radiation field often becomes red, dry, and tender, similar to a sunburn that develops gradually over the course of treatment.
Cognitive Effects (“Chemo Brain”)
Many people undergoing chemotherapy notice problems with memory, concentration, and mental sharpness. This cluster of symptoms, commonly called chemo brain, affects processing speed, working memory, visual memory, and verbal memory. You might find yourself struggling to multitask, losing your train of thought mid-sentence, or taking longer to make decisions.
For most people, these cognitive changes improve after treatment ends. But roughly one third of patients report symptoms lasting months to as long as 5 to 10 years after finishing chemotherapy. The effects can be subtle enough that standard cognitive tests miss them, yet noticeable enough to interfere with work performance and daily planning.
Fertility and Reproductive Health
Both chemotherapy and radiation to the pelvic region can impair fertility in men and women. Chemotherapy drugs can reduce sperm count or damage eggs, while pelvic radiation can harm reproductive organs directly.
If you’re of childbearing age and plan to have children in the future, fertility preservation is ideally discussed before treatment starts. For men, sperm banking is the most established option. Samples are collected, frozen, and stored indefinitely for later use with in vitro fertilization. For men who can’t produce a sample through ejaculation, surgical retrieval from testicular tissue is an alternative. During pelvic radiation, testicular shielding can reduce scatter radiation reaching the testes. Women have parallel options, including egg freezing and embryo freezing, which are typically coordinated through a fertility clinic before the first treatment cycle.
Long-Term and Late Effects
Some side effects don’t appear until months or years after treatment ends. Radiation, in particular, carries a small but real risk of causing a new, unrelated cancer in or near the area that was treated. Most of these secondary cancers develop only after decades, and the risk increases with higher radiation doses and larger treatment areas. Younger patients face a higher lifetime risk simply because they have more years ahead in which a secondary cancer could develop.
The risk per unit of radiation dose is actually lower for cancer patients receiving fractionated treatment (small daily doses spread over weeks) than it is for people exposed to a single large dose, such as atomic bomb survivors. Still, the cumulative effect matters. Most radiation-related secondary cancers arise within the area that received the highest dose, though distant organs like the lungs can also be affected even at much lower doses.
Managing Nausea and Other Symptoms
Nausea is one of the most dreaded side effects, but it’s also one of the most treatable. Modern anti-nausea medications work by blocking the brain’s vomiting signals through several different pathways. Your oncologist will typically prescribe a combination of these drugs tailored to how nausea-inducing your specific chemotherapy regimen is. Most patients receive anti-nausea medication before each infusion, with additional doses to take at home for the days following treatment.
For fatigue, no single medication works reliably. Light physical activity, even short daily walks, has shown more consistent benefits than rest alone. Keeping a predictable sleep schedule and pacing your energy throughout the day helps many people manage the worst stretches. Fatigue from radiation tends to build gradually over the course of treatment and can linger for weeks to months afterward before slowly improving.
Skin irritation from radiation responds well to gentle, fragrance-free moisturizers applied to the treatment area. Your radiation team will give you specific guidance on what products to use and avoid, since some creams can interfere with treatment delivery if applied too close to your session time.