What is Naltrexone and How Does it Work?
Naltrexone is a medication primarily used to manage alcohol use disorder (AUD) and opioid use disorder (OUD) [1.5.1]. It is classified as an opioid antagonist, which means it works by blocking the effects of opioids at the receptor sites in the brain [1.3.5]. For individuals with OUD, this blockade prevents the euphoric feelings associated with opioid use. In the case of AUD, naltrexone is thought to reduce the rewarding effects of alcohol by blocking the body's naturally produced opioids (endorphins) that are released during consumption [1.3.5].
Approved for medical use in the U.S. in 1984, naltrexone is available as an oral tablet (Revia) and a monthly intramuscular injection (Vivitrol) [1.5.1, 1.9.3]. It is not an opioid, is not addictive, and does not cause withdrawal symptoms upon discontinuation [1.3.5, 1.9.4]. Treatment with naltrexone is most effective when combined with a comprehensive program that includes counseling and social support [1.9.1].
The Critical Abstinence Period
It is crucial that individuals are free from opioids for a period of 7 to 10 days before starting naltrexone [1.2.3, 1.9.4]. Taking naltrexone while opioids are still in the system can precipitate acute and severe opioid withdrawal symptoms because the medication will displace the opioids from their receptors [1.2.3]. These withdrawal-like symptoms can include abdominal cramps, diarrhea, and vomiting [1.2.3, 1.9.3].
Can Naltrexone Cause Diarrhea? The Pharmacological Reason
Yes, naltrexone can cause diarrhea. Gastrointestinal complaints, including diarrhea and abdominal cramping, are among the most common side effects reported with naltrexone use [1.2.3, 1.2.6]. In fact, diarrhea is listed as a potential side effect in less than 10% of users, though other sources categorize it as simply 'common' [1.2.3, 1.2.4].
The reason for this side effect lies in its mechanism of action. Opioid receptors are not only present in the brain but also in the gastrointestinal tract. The blockade of these µ-opioid receptors in the gut leads to increased gastrointestinal motility [1.2.3]. This increase in movement can result in symptoms analogous to opioid withdrawal, such as abdominal cramps and diarrhea [1.2.3]. This effect can occur even in individuals who are not dependent on opioids [1.2.3].
How Long Does It Last?
For most people, side effects like diarrhea are temporary and tend to occur within the first few days or weeks of starting the medication [1.2.1, 1.7.3]. These initial side effects typically subside on their own as the body adjusts to the medication, often within a few days to a couple of weeks [1.7.1, 1.7.4]. However, if diarrhea is severe or persists, it's important to consult a healthcare provider [1.2.1, 1.9.1].
Managing Naltrexone-Induced Diarrhea
While often mild and transient, there are several strategies to help manage diarrhea and other gastrointestinal issues caused by naltrexone. Always consult your doctor before trying new remedies or changing your medication routine [1.4.1].
Practical Management Tips
- Start with a Low Dose: Healthcare providers often start patients on a lower dose (e.g., 25 mg) and gradually increase it to the standard 50 mg daily dose. This allows the body to acclimate more slowly, reducing the intensity of side effects [1.2.1, 1.4.1].
- Stay Hydrated: Diarrhea can lead to dehydration. Drinking plenty of fluids, especially water or electrolyte-rich drinks, is crucial to replace lost fluids and can help alleviate associated symptoms like headaches [1.4.1].
- Dietary Adjustments: Avoid high-fat meals and caffeine, which can exacerbate diarrhea [1.4.2]. Eating smaller, more frequent meals throughout the day might also be easier on the digestive system [1.4.1]. Taking naltrexone with food, such as complex carbohydrates, can help minimize general stomach upset [1.4.1, 1.4.4].
- Adjust Dosing Time: If side effects are particularly bothersome, talk to your doctor about changing the time of day you take your dose. For example, if fatigue is also an issue, taking it at night might be beneficial [1.4.1].
- Over-the-Counter (OTC) Options: While anti-diarrheal medications might seem like a logical choice, it's important to know that naltrexone can decrease the effectiveness of certain anti-diarrheal products [1.9.2]. Always consult your healthcare provider before taking any OTC medications [1.4.2].
Comparison with Other Medications for Alcohol Use Disorder
Naltrexone is one of several medications used to treat AUD. Acamprosate and disulfiram are other common options. Their side effect profiles, particularly regarding GI issues, differ.
| Medication | Common GI Side Effects | Mechanism Notes |
|---|---|---|
| Naltrexone | Nausea, abdominal pain, diarrhea, constipation [1.2.3, 1.2.4, 1.6.1] | Blocks opioid receptors, increasing gut motility [1.2.3]. |
| Acamprosate | Diarrhea (primary GI effect), nausea, abdominal discomfort [1.6.1, 1.6.3] | Believed to restore balance to brain neurotransmitters affected by alcohol [1.6.1]. |
| Disulfiram | (No primary GI side effects) | Creates an intensely unpleasant reaction to alcohol (nausea, vomiting, flushing). |
Acamprosate is also known for causing diarrhea [1.6.1, 1.6.5]. Naltrexone may have a slightly larger effect size in reducing heavy drinking, while acamprosate may be more effective for maintaining complete abstinence [1.6.4]. The choice between them depends on a patient's medical history, drinking patterns, and tolerance for potential side effects [1.6.6].
When to See a Doctor
You should contact your healthcare provider if diarrhea is severe, persists for more than a few weeks, or is accompanied by other serious symptoms [1.2.1, 1.7.2]. Seek immediate medical attention if you experience severe vomiting and diarrhea, confusion, blurred vision, or yellowing of the skin or eyes, as these could be signs of a more serious issue like liver damage [1.2.5, 1.9.1].
Conclusion
Diarrhea is a recognized and relatively common side effect of naltrexone, stemming from its function as an opioid antagonist that affects receptors in the digestive tract [1.2.3]. For most individuals, this side effect is mild and resolves within the first few weeks of treatment as the body adjusts [1.7.3]. Management strategies like starting with a low dose, staying hydrated, and taking the medication with food can help mitigate discomfort [1.4.1]. As with any medication, open communication with a healthcare provider is key to managing side effects effectively and ensuring the treatment is both safe and successful.
For more information from an authoritative source, you can visit the Substance Abuse and Mental Health Services Administration (SAMHSA).
