Radiation therapy and constipation: what to expect and how to manage

Constipation can occur during radiation therapy, especially when the pelvis or nearby abdominal areas are treated, although diarrhea is more common. Radiation can inflame tissues in the bowel and rectum, which may lead to straining with little stool output or a feeling of incomplete bowel movements. ^{[1] [2]} This inflammation can change bowel activity and cause symptoms such as rectal pressure, cramping, and difficulty passing stool. ^{[1] [3]}

Why constipation can happen

• Tissue inflammation from radiation can make bowel movements harder to pass, even if stool volume is low. People may feel the urge to go but strain without results due to rectal discomfort and swelling. ^[1]
• Pelvic radiation commonly affects the rectum and bowel function, leading to changes in bowel patterns that can include both diarrhea and episodes of difficult stool passage. ^{[1] [2]}
• These bowel changes typically improve within 2 to 4 weeks after completing radiation, as irritation settles and bowel habits return toward baseline. ^[4]

First-line self-care strategies

Gentle, consistent habits can reduce straining and support regularity. ^[5]

• Hydration: Drink plenty of liquids throughout the day; warm drinks (like hot tea) can help trigger bowel movements. ^[6]
• Routine timing: Try going to the bathroom at the same time daily, and don’t delay when you feel the urge. ^[5]
• Activity: Increase physical activity as you’re able, which helps bowel motility. Discuss any new exercise with your care team if needed. ^[6]
• Avoid straining: Excessive pushing can worsen rectal swelling during pelvic radiation; aim for relaxed, unhurried toileting. ^[7]
• Comfort measures: Sitz baths with cool or lukewarm water can soothe rectal discomfort after bowel movements. ^[7]

Eating tips during pelvic radiation

Diet may need temporary adjustment based on your symptoms. If you have bloating or watery stools, a low-fiber, low-fat, low-lactose plan is often advised until your bowel pattern normalizes. ^[4]

• When bowels are irritated (cramping, straining with little stool), a low-fiber, low-fat, low-lactose diet can reduce bowel workload and rectal irritation. ^[4]
• Hydrate adequately to keep stools soft and replace losses if diarrhea alternates with difficult passage. Aim for steady fluid intake during the day. ^[8]
• Reintroduce regular foods gradually once bowel movements return to your usual pattern. ^[4]
• If your main issue is classic constipation (hard, infrequent stools), include more high‑fiber foods, fluids, and regular meals to stimulate the bowels, while monitoring how your pelvis and bowel tolerate fiber during radiation. ^[6]

Over-the-counter medicines

Many people benefit from simple bowel medicines; your care team can help choose and dose them. ^[5]

• Stool softeners (e.g., docusate) can make stool easier to pass and reduce straining. ^[5]
• Osmotic laxatives (e.g., polyethylene glycol) draw water into the bowel to soften stool and improve frequency. ^[5]
• Stimulant laxatives (e.g., senna, bisacodyl) can be used short term if other options are not enough, under guidance. ^[5]
• Always follow your provider’s instructions and inform them if you have diarrhea after taking these medicines. ^[5]

When to contact your care team

Certain symptoms need prompt attention to keep you safe. ^[5]

• No bowel movement for 3 days despite home measures. ^[5]
• Bleeding during bowel movements or new rectal pain that does not improve. ^[5]
• Diarrhea after taking constipation medicines or mixed symptoms that are hard to manage at home. ^[5]

What to expect over time

Bowel symptoms from pelvic radiation usually improve within 2–4 weeks after the last treatment, and your diet can be relaxed gradually as you recover. Keep following supportive guidelines until your usual pattern returns. ^[4] If symptoms persist or worsen, your oncology team can adjust your plan and recommend targeted treatments to protect bowel comfort and function. ^[3]