Chemotherapy treatments often result in unintended side effects, including changes to normal digestive function. Chemotherapy does cause constipation; it is a common side effect experienced by many patients receiving anti-cancer agents. Constipation is defined as having infrequent bowel movements, often fewer than three per week, characterized by hard, dry stools that are difficult to pass. This is a manageable condition that cancer care teams anticipate and treat proactively.
How Chemotherapy Affects Bowel Function
The primary cause of chemotherapy-induced constipation is the direct effect of certain drugs on the body’s nervous system, specifically the enteric nervous system (ENS). The ENS controls the movement of the digestive tract, a process known as peristalsis. Certain chemotherapy agents are neurotoxic, meaning they can damage the nerve cells within the intestinal wall. This damage slows the signaling between the nerves and the intestinal muscles, leading to decreased peristalsis. When these rhythmic muscular contractions slow down, the stool remains in the large intestine longer, allowing more water to be absorbed.
Drugs known for this neurotoxic effect include the vinca alkaloids, such as vincristine, and platinum-based drugs like cisplatin. For patients receiving these specific agents, the incidence of chemotherapy-induced constipation can be quite high, sometimes affecting up to 90% of those treated. The severity is often related to the specific drug combination and the dose administered.
Other Medications That Increase Constipation Risk
Chemotherapy rarely acts in isolation, and the risk of constipation is compounded by other medications essential to cancer care. Opioid pain medications are a major contributor, causing opioid-induced constipation (OIC), which affects over half of all cancer patients taking them. Opioids bind to mu-opioid receptors throughout the gastrointestinal tract, actively inhibiting the forward movement of the bowel. This action decreases intestinal secretions and impairs muscle contraction, leading to severe constipation often resistant to standard laxatives.
Anti-nausea drugs, or antiemetics, are another class of medications that can inadvertently slow the bowel. Specifically, 5-HT3 receptor antagonists, such as ondansetron or granisetron, block serotonin receptors to prevent vomiting. Since these same serotonin receptors stimulate gut motility, blocking them contributes to a sluggish digestive system.
External factors common during cancer treatment further increase the risk of constipation. Reduced physical activity and fatigue often lead to prolonged periods of rest, which naturally slows intestinal movement. Side effects like nausea and loss of appetite can lead to decreased fluid and fiber intake, causing dehydration and a lack of bulk in the stool. Even oral iron supplements, often prescribed to combat anemia, can cause constipation by making the gut absorb more water or by altering the gut microbiome.
Strategies for Prevention and Management
Managing constipation requires a proactive and multi-faceted approach, often beginning with simple lifestyle adjustments. Increasing fluid intake is the most important step, with a goal of drinking eight to ten cups of clear liquids daily, such as water, broth, or prune juice. Gentle physical activity, like short walks, can also help stimulate the intestinal muscles to promote movement.
Dietary fiber is beneficial but must be introduced slowly and with sufficient fluid to prevent worsening the blockage. High-fiber foods, such as whole grains, fresh fruits, and vegetables, add bulk to the stool, but in cases of severe constipation, a healthcare team may advise against them until the blockage is cleared.
Over-the-counter medications typically fall into three categories based on their mechanism of action. Osmotic laxatives, such as polyethylene glycol (PEG), work by drawing water into the colon, softening the stool and promoting a bowel movement. Stool softeners like docusate sodium increase the amount of water and fat the stool absorbs, making it easier to pass. Stimulant laxatives, such as senna or bisacodyl, actively cause the intestinal muscles to contract, pushing the stool forward. These are often used when movement is severely impaired, particularly in cases of opioid-induced constipation.
For OIC that does not respond to standard laxatives, specialized prescription drugs known as Peripherally Acting Mu-Opioid Receptor Antagonists (PAMORAs) may be used. These agents, including methylnaltrexone and naloxegol, specifically block the opioid receptors in the gut without affecting the pain-relieving effects in the brain.
It is important to contact the cancer care team immediately if constipation persists, as delayed treatment can lead to serious complications like fecal impaction or bowel obstruction. Warning signs include not having a bowel movement for three or more days, experiencing severe abdominal pain or cramping, or having any vomiting. Any sign of blood in the stool or black, tarry stools also warrants immediate medical attention.