Naltrexone can cause constipation, but for most people taking it, constipation is one of the less common side effects. How likely it is depends heavily on which form you take, since the same drug turns up in very different medicines. When constipation does show up, it tends to be mild and often fades as the body adjusts.
How Often Naltrexone Causes Constipation
Constipation appears on naltrexone’s list of possible side effects, grouped with the less common ones. One estimate puts it at about 1 to 10 people in 100, and it is fairly rare at the standard 50 mg dose used for alcohol.1Choose Your Horizon. Naltrexone Constipation: Rare Side Effect That May Not Be From the Medication In placebo-controlled trials of oral naltrexone, the side effects that clearly showed up more than on a dummy pill were nausea, vomiting, dizziness, sleepiness, sweating and reduced appetite. Constipation was not among them.2PubMed Central. Serious adverse events reported in placebo randomised controlled trials of oral naltrexone: a systematic review and meta-analysis Those common effects were mild.3PubMed Central. Serious adverse events reported in placebo randomised controlled trials of oral naltrexone: a systematic review and meta-analysis
A range like 1 to 10 in 100 means most people on naltrexone will never notice constipation, though some will. The wording on a label describes how often something was reported, not whether the drug caused it in each person.
Many people on a placebo also report constipation, so not every case is caused by the drug. Your diet, fluids, activity and other medicines all affect your bowels at the same time.
It also helps to know what counts as constipation. It generally means fewer than three bowel movements a week, or hard stools that are difficult to pass. A day or two without going after starting a new pill is not automatically constipation. Naltrexone can reduce appetite, and eating less means less stool, which can feel like being backed up. Tracking how often you go for a week or two gives a clearer picture than one bad day.
Constipation that starts soon after the first dose or a dose increase and keeps recurring is more suggestive than a single bad day. A written log gives you and your prescriber something concrete to judge. Even mild constipation is worth dealing with, because stomach and bowel complaints were linked to people taking naltrexone less reliably.4PubMed. Adverse effects of oral naltrexone: analysis of data from two clinical trials
Why the Type of Naltrexone You Take Matters
Naltrexone is also sold combined with bupropion as a prescription weight-loss pill. Common side effects of that combination include nausea, constipation, vomiting, dizziness and dry mouth.5PubMed Central. Naltrexone/Bupropion: An Investigational Combination for Weight Loss and Maintenance
Each tablet contains bupropion as well as naltrexone, so constipation may not come from the naltrexone part alone. The dose is usually stepped up over several weeks, and stomach and bowel effects often show up during that build-up.
Low-dose naltrexone is a much smaller dose, often a few milligrams, prescribed off-label for chronic pain and other conditions. In people with chronic pain, it was generally well tolerated, with vivid dreams, diarrhea and headache the most common effects.6PubMed Central. Low-dose naltrexone’s utility for non-cancer centralized pain conditions: a scoping review Constipation on low-dose naltrexone is unusual, and diarrhea is more likely.
If you aren’t sure which product you have, the details are easy to check. Standard naltrexone for alcohol or opioid use disorder is usually a 50 mg tablet taken once a day. Low-dose naltrexone is often made up by a compounding pharmacy, in doses far below 50 mg, as capsules or liquid. The weight-loss product combines naltrexone with bupropion in one extended-release tablet, and its label names both drugs. Your pharmacy label, or a quick question to the pharmacist, will settle it.
How Naltrexone Can Affect Your Bowels
The gut runs partly on the body’s own opioid signals. Nerves in the gut wall release natural opioid-like chemicals, which act on opioid receptors that control movement and fluid in the bowel.7PubMed Central. Opioid receptors in the gastrointestinal tract Those signals can quiet both the nerves that push contents along and the nerves that relax the muscle.8PubMed Central. Opioid receptors in the gastrointestinal tract Naltrexone blocks opioid receptors, including the ones in the gut.9Choose Your Horizon. Naltrexone Constipation: Rare Side Effect That May Not Be From the Medication Blocking them can change how the bowel moves, which is how naltrexone can slow digestion in some people.10Choose Your Horizon. Naltrexone Constipation: Rare Side Effect That May Not Be From the Medication
This is different from the constipation caused by opioid painkillers. Those drugs switch the gut’s opioid receptors on, which reliably slows the bowel and makes constipation a very common problem. Naltrexone blocks the receptors rather than activating them, so it does not usually produce that strong slowing.
The effect also stays small because very little swallowed naltrexone reaches the bloodstream intact, under 1% in animal tests.11International Journal of Pharmaceutics. Buccal and oral bioavailability of naloxone and naltrexone in rats Still, swallowed opioid blockers act directly on the gut wall.12PubMed. Orally administered opioid antagonists reverse both mu and kappa opioid agonist delay of gastrointestinal transit in the guinea pig
What to Do About Constipation on Naltrexone
Lifestyle steps come first. Advice for drug-related constipation starts with more fiber and fluid and more movement or exercise.13PubMed Central. Management of Opioid-Induced Constipation and Bowel Dysfunction: Expert Opinion of an Italian Multidisciplinary Panel Add fiber gradually, drink water through the day and take a daily walk. Side effects like these often ease as your body adjusts to the medicine, and low-dose naltrexone side effects tended to fade as treatment continued.14PubMed Central. Low-dose naltrexone’s utility for non-cancer centralized pain conditions: a scoping review
Add one high-fiber food a day, such as oats, beans, prunes or berries, rather than changing your whole diet at once. Try to sit on the toilet at the same time each day, often after breakfast when the bowel is most active.
For constipation caused by medicines, the recommended first choice is an osmotic laxative, preferably polyethylene glycol, or a stimulant laxative.15Journal of Neurogastroenterology and Motility. Opioid-induced Constipation: Old and New Concepts in Diagnosis and Treatment An osmotic laxative draws water into the stool to soften it, and polyethylene glycol is sold over the counter, for example as MiraLAX. If one laxative is not enough, a second one that works a different way can be added.16PubMed Central. Management of Opioid-Induced Constipation and Bowel Dysfunction: Expert Opinion of an Italian Multidisciplinary Panel Ask a pharmacist which over-the-counter option suits you before combining products.
A few mistakes are common. Using stimulant laxatives every day without advice isn’t ideal, so check with a pharmacist or prescriber if you need them regularly. Assuming it must be the naltrexone can mean missing another cause, such as iron pills.
Don’t stop naltrexone on your own. Tell your prescriber about constipation instead of skipping doses, since timing or dose can often be adjusted.
When Constipation May Not Be From Naltrexone
Opioids are a much stronger cause. Opioid painkillers and drugs commonly cause constipation, along with nausea, bloating and swelling of the belly.17PubMed. The narcotic bowel syndrome: a recent update Naltrexone blocks opioids and can trigger sudden opioid withdrawal in someone who still has opioids in their body.18PubMed. Constitutive opioid receptor activation: a prerequisite mechanism involved in acute opioid withdrawal Withdrawal brings fast heart rate, restlessness, yawning, runny nose and vomiting.19PubMed. Constitutive opioid receptor activation: a prerequisite mechanism involved in acute opioid withdrawal Never take an opioid painkiller, or any other opioid-based medicine such as some cough or diarrhea remedies, with naltrexone without your prescriber’s guidance.
Specialist constipation drugs are a different thing. Some related drugs, such as methylnaltrexone, block opioid receptors only in the gut and do not reach the brain.20PubMed. Methylnaltrexone for the treatment of opioid-induced constipation These are used for constipation caused by opioid painkillers, often after laxatives have not worked.21Journal of Neurogastroenterology and Motility. Opioid-induced Constipation: Old and New Concepts in Diagnosis and Treatment
Several everyday causes are worth checking too. Iron supplements, antacids containing aluminum or calcium, some antidepressants and some antihistamines commonly cause constipation. Stopping heavy drinking often changes eating and drinking patterns, which can change your stools. Eating less, which can happen on naltrexone or the weight-loss pill, means less fiber and less stool. Low activity, stress and dehydration add up too, so several small factors may be working together.
Constipation that began before you started naltrexone, or after another change such as a new medicine, points elsewhere. Timing is a clue rather than proof, because several changes often happen together.
When to Call Your Doctor
Most cases are mild, and mild side effects that continue or bother you are worth raising with your prescriber. Severe belly pain, vomiting, blood in the stool, black stools or being unable to pass gas need prompt care. Severe belly pain with vomiting in someone who has recently used opioids can signal withdrawal or another problem, not simple constipation.22PubMed. The narcotic bowel syndrome: a recent update Don’t treat these with laxatives at home; get checked.
When you do call, a few details make the conversation more useful. Say which form and dose of naltrexone you take and when you started. Describe how often you are going now compared with before. List every other medicine and supplement you take, including over-the-counter ones. Mention what you have already tried and whether it helped.
Your prescriber may suggest taking the dose with food or at a different time of day. With the weight-loss pill, they may slow the dose increase. They may recommend a specific laxative routine that is safe alongside your treatment, and they may also check whether another medicine is the real cause.