When men receive radiation therapy for prostate cancer, the treatment can sometimes irritate the rectum (the end of the large intestine). Researchers studied whether following bowel preparation instructions—like taking a specific laxative called Milk of Magnesia and eating a special diet—could reduce these side effects. They tracked 7 prostate cancer patients over several months and found that patients who took their bowel preparation medicine as instructed experienced fewer rectal problems. This suggests that following your doctor’s preparation instructions carefully is an important but often overlooked part of getting radiation therapy safely.

The Quick Take

  • What they studied: Whether following bowel preparation instructions (taking a laxative and eating a special diet) reduces rectal damage in men receiving radiation therapy for prostate cancer
  • Who participated: 7 men with prostate cancer (average age 64) who were receiving radiation therapy at a hospital in Pakistan between July and December 2024
  • Key finding: Men who took their bowel preparation medicine (Milk of Magnesia) as instructed had significantly fewer rectal problems compared to those who didn’t follow the instructions, even though only about 1 in 4 patients actually followed the protocol well
  • What it means for you: If you’re getting radiation therapy for prostate cancer, carefully following your doctor’s bowel preparation instructions—especially taking your prescribed laxative—may help protect you from painful rectal side effects. However, this is a small study, so talk with your doctor about what’s best for your situation.

The Research Details

This was a prospective observational study, which means researchers followed patients forward in time and observed what happened without randomly assigning them to different treatment groups. Seven prostate cancer patients receiving radiation therapy were enrolled, and researchers tracked them from July through December 2024. Before each radiation treatment, patients were supposed to follow a bowel preparation protocol: eating a special diet that reduces gas, taking Milk of Magnesia (a common laxative), and drinking 500 milliliters of water 30 minutes before imaging scans. Researchers used special CT imaging (called cone beam CT) taken daily to monitor the size and position of the rectum during treatment. They collected information about whether patients followed the instructions, measured the rectum’s size and movement, and tracked any side effects patients experienced.

The rectum sits very close to the prostate, so radiation aimed at the prostate can accidentally damage the rectum, causing pain, bleeding, and other problems. If doctors can predict and control how much the rectum moves and changes size during treatment, they can aim the radiation more precisely and reduce these side effects. Understanding whether simple preparation steps like taking a laxative actually help is important because it could be an easy way to improve treatment safety.

This study has some important limitations to understand: Only 7 patients were studied, which is a very small number and makes it hard to be confident in the results. The study was observational rather than randomized, meaning patients weren’t randomly assigned to follow or not follow the protocol—some chose to follow it and others didn’t. The study was conducted at a single hospital, so results may not apply everywhere. Additionally, patient compliance was very poor (only about 25% of patients followed the bowel preparation instructions), which makes it harder to draw strong conclusions. Despite these limitations, the findings suggest a real benefit to following instructions.

What the Results Show

The main finding was that patients who took Milk of Magnesia as instructed experienced significantly fewer rectal side effects (toxicity) compared to those who didn’t take it. This relationship was statistically significant, meaning it’s unlikely to be due to chance. Interestingly, following the special diet alone didn’t show the same clear benefit in the statistical analysis, even though it seemed helpful. The researchers found that the rectum’s size and position varied considerably from day to day during treatment, which is why the preparation protocol matters—it helps keep the rectum smaller and more stable. The average radiation dose to the rectum increased over the course of treatment, which is expected as patients receive multiple radiation sessions.

The study also found that individual patients had different amounts of rectal movement and size changes during their treatment course. Some patients’ rectums stayed relatively stable while others varied more. The researchers were able to track these changes using daily imaging, which helped them understand how important it is to keep the rectum as consistent as possible. The study confirmed that the radiation doses patients received (7800 cGy over 39 treatment sessions) were appropriate for prostate cancer treatment.

Previous research has suggested that bowel preparation can help reduce radiation side effects, but this study provides more specific evidence about Milk of Magnesia’s role. The finding that medication compliance matters more than dietary compliance is somewhat new and suggests that doctors should emphasize the importance of taking prescribed laxatives. The study’s focus on daily imaging to track rectal changes represents modern precision medicine approaches that weren’t available in older studies.

The biggest limitation is the very small sample size of only 7 patients, which makes it impossible to be highly confident in the results. A much larger study would be needed to confirm these findings. Second, only about 1 in 4 patients actually followed the bowel preparation instructions well, which limits how much we can learn about what happens when people do follow instructions. Third, this was a single-center study at one hospital in Pakistan, so results might be different in other countries or hospitals with different patient populations. Fourth, the study didn’t have a control group of patients who didn’t receive any bowel preparation, so we can’t compare to that scenario. Finally, the study period was relatively short (6 months), so we don’t know about long-term effects.

The Bottom Line

If you’re receiving radiation therapy for prostate cancer, follow your doctor’s bowel preparation instructions carefully, especially taking any prescribed laxatives like Milk of Magnesia. This appears to reduce painful rectal side effects. However, this recommendation is based on a small study, so discuss with your radiation oncologist whether this approach is right for you. The evidence is moderate in strength—it suggests a real benefit, but larger studies would provide stronger confirmation.

This research is most relevant for men with prostate cancer who are about to receive radiation therapy. It’s also important for radiation oncologists and medical teams who treat these patients. If you have other medical conditions affecting your bowel or take medications that interact with laxatives, discuss this with your doctor before starting bowel preparation. This research doesn’t apply to people receiving other types of cancer treatment or those not receiving radiation therapy.

You should expect to follow bowel preparation instructions throughout your entire course of radiation therapy, which typically lasts several weeks. The benefits in reducing rectal side effects should develop gradually over the treatment period. Some patients may notice improvements in bowel comfort within the first few weeks, while others may take longer. Long-term benefits in preventing serious rectal problems may take months to become apparent.

Want to Apply This Research?

  • Set daily reminders to take your bowel preparation medication at the same time each day, and track compliance by checking off each day you took it. Also track any rectal symptoms (discomfort, bleeding, urgency) on a simple 0-10 scale to see if following instructions correlates with fewer symptoms.
  • Use your app to create a daily checklist: (1) Take Milk of Magnesia or prescribed laxative at the scheduled time, (2) Follow the recommended diet by logging meals, (3) Drink the required water 30 minutes before your radiation appointment. Set phone notifications for each step to build the habit.
  • Track three metrics weekly: medication compliance percentage, rectal symptom severity score, and bowel movement consistency. Create a simple graph showing how compliance correlates with symptom improvement. Share this data with your radiation oncology team at each appointment to demonstrate adherence and discuss any side effects.

This research describes findings from a small study of 7 patients and should not replace medical advice from your healthcare team. Bowel preparation protocols vary by hospital and individual patient factors. Before starting or changing any bowel preparation regimen, including taking Milk of Magnesia or other laxatives, consult with your radiation oncologist or physician. This is especially important if you have other medical conditions, take other medications, or have had previous bowel surgery. The findings suggest potential benefits but are not definitive proof. Always follow your doctor’s specific instructions for your individual treatment plan.