Naltrexone is a medication primarily prescribed to support individuals managing Opioid Use Disorder (OUD) and Alcohol Use Disorder (AUD). It functions by blocking opioid receptors in the brain. Naltrexone is also utilized in a combination product with bupropion for chronic weight management. Given its systemic effects, a common question arises regarding its impact on the digestive system. This article addresses whether Naltrexone can cause constipation and explores the science behind this reported side effect.
Naltrexone and Gastrointestinal Function
Naltrexone can cause constipation, though the frequency depends on the dosage and whether it is used alone or in a combination product. When prescribed alone for OUD or AUD, constipation is a less common adverse effect, typically reported by under 10% of patients in clinical trials. Digestive issues such as nausea and abdominal pain are generally observed more frequently than constipation with the stand-alone medication.
The incidence rate, however, is notably higher when Naltrexone is combined with bupropion for weight management. In this combination therapy, constipation is a common side effect, sometimes affecting 15% to 24% of users. This significant difference suggests that the effect is often amplified by the concurrent medication, or that it is a direct result of the combined pharmacological action. Many people taking Naltrexone do not experience this particular gastrointestinal change.
The Science Behind the Effect
The mechanism by which Naltrexone acts makes the occurrence of constipation paradoxical. Naltrexone is an opioid antagonist, blocking the mu-opioid receptors highly concentrated in the gut’s enteric nervous system. When opioids bind to these receptors, they slow down peristalsis (the muscle contractions that move food through the intestines) and reduce fluid secretion, resulting in hard, dry stools.
Because Naltrexone blocks these receptors, its primary action should theoretically be to prevent or relieve opioid-induced constipation. This is the principle behind using other peripheral opioid antagonists to treat severe constipation caused by pain medications. The reported constipation must therefore be attributed to effects beyond Naltrexone’s primary antagonistic action alone.
When Naltrexone is combined with bupropion, the latter medication likely contributes significantly to the digestive slowing. Bupropion has mild anticholinergic effects, which can slow down gut motility and reduce secretions, leading to the common side effects of dry mouth and constipation. For Naltrexone used alone, the reported constipation may result from general digestive upset, such as nausea and appetite loss, or indirect systemic changes that influence bowel regularity.
Managing Constipation While on Naltrexone
Managing constipation effectively requires a proactive approach centered on lifestyle adjustments and appropriate over-the-counter interventions. The first line of action involves increasing fluid intake, as dehydration is a common factor in hard stools and can be exacerbated by medications like Naltrexone or bupropion that may cause dry mouth. Consuming plenty of water throughout the day helps keep the intestinal contents soft and easier to pass.
Dietary modifications should focus on increasing fiber consumption, paying close attention to the type of fiber consumed. Soluble fiber, found in foods like oats, apples, and beans, absorbs water and forms a gel-like substance, which adds bulk to the stool while keeping it soft. Insoluble fiber, found in whole grains and vegetables, adds mass and speeds up the transit of food through the digestive tract.
Regular physical activity is also highly beneficial because movement stimulates the muscles of the intestines, supporting normal peristaltic motion. Even a moderate daily walk can help encourage regular bowel movements and prevent stool from becoming stagnant in the colon. Establishing a consistent daily routine for attempting a bowel movement, such as after a meal, can also help train the body’s natural reflexes.
If lifestyle changes are insufficient, over-the-counter laxatives can provide relief, but it is important to choose the right type. Osmotic laxatives, such as polyethylene glycol, work by drawing water into the colon, softening the stool and promoting movement. Stool softeners, like docusate sodium, increase the amount of water the stool absorbs, making it easier to pass without stimulating the bowels directly. Always consult with a pharmacist or healthcare provider before starting any new supplement or laxative to ensure it is appropriate for use with Naltrexone.
Symptoms Requiring Medical Attention
While mild constipation can often be managed at home, certain symptoms serve as warning signs that require immediate medical consultation. Severe, sharp, or persistent abdominal pain that does not improve or worsens over time may indicate a more serious issue than simple constipation. The inability to pass gas or stool for an extended period, particularly if accompanied by increasing abdominal swelling or a feeling of fullness, could signal a bowel obstruction or impaction.
Vomiting, especially if the vomit contains material that looks like stool, indicates a significant digestive blockage requiring urgent medical attention. Any observation of blood in the stool, whether bright red or dark and tarry, should also prompt an immediate discussion with a healthcare provider. Patients should never abruptly stop taking Naltrexone without first consulting their prescribing physician, even if they are experiencing bothersome digestive side effects.