What is Amitriptyline?
Amitriptyline is a medication classified as a tricyclic antidepressant (TCA) [1.2.3]. While originally developed to treat depression by increasing levels of norepinephrine and serotonin in the brain, its use has expanded significantly [1.3.1]. Today, doctors prescribe it for a variety of conditions, including anxiety, insomnia, migraine prevention, and chronic nerve pain [1.3.1]. Due to its effects on the gastrointestinal (GI) tract, it is also used off-label to manage functional gut disorders like Irritable Bowel Syndrome (IBS) [1.4.3, 1.4.5]. Amitriptyline works by blocking the action of several neurotransmitters, including acetylcholine, which plays a key role in regulating bodily functions like digestion [1.4.3].
The Primary Culprit: Anticholinergic Effects and Constipation
The most frequently reported change in bowel movements for those taking amitriptyline is constipation [1.2.1]. This occurs because amitriptyline has strong anticholinergic properties [1.2.3]. Anticholinergic medications work by blocking the neurotransmitter acetylcholine. In the digestive system, acetylcholine is responsible for stimulating the muscular contractions that move waste through the intestines [1.4.3]. By blocking this action, amitriptyline slows down how fast food moves through your gut [1.4.2]. This delayed transit time allows more water to be absorbed from the stool, making it harder and more difficult to pass [1.4.3]. The intestinal secretions that lubricate the passage of feces can also become drier, further contributing to constipation [1.4.3]. This side effect can be dose-dependent, meaning higher doses may increase the risk [1.4.1].
Can Amitriptyline Cause Diarrhea?
Though less common than constipation, diarrhea is also a possible side effect of amitriptyline [1.3.2, 1.4.8]. The exact reasons for this are less clear than the mechanism for constipation. For some individuals, the body's initial adjustment to the medication can manifest as diarrhea. In other cases, it could be a symptom of a more serious condition like serotonin syndrome, which can occur with antidepressant use and includes symptoms like agitation, fast heart rate, muscle twitching, vomiting, and diarrhea [1.3.4]. It's crucial to report any persistent or severe diarrhea to a healthcare provider.
A Dual Role: Treating Irritable Bowel Syndrome (IBS)
Interestingly, the very properties that cause constipation make amitriptyline a useful treatment for certain types of IBS. Specifically, for Irritable Bowel Syndrome with Diarrhea (IBS-D), low-dose amitriptyline can be effective [1.6.3]. By slowing down gastrointestinal transit, it helps reduce the frequency of bowel movements and can improve stool consistency [1.4.5, 1.7.2]. A meta-analysis of multiple studies confirmed that amitriptyline was effective in treating IBS, particularly for patients with the IBS-D subtype, by improving diarrhea and reducing overall symptom severity scores [1.3.6]. The ATLANTIS trial, a large study conducted in the UK, also found that low-dose amitriptyline was safe, well-tolerated, and significantly improved IBS symptoms compared to a placebo [1.6.5].
However, for individuals with IBS with constipation (IBS-C), amitriptyline should be used with caution, as it can worsen symptoms [1.4.1].
Comparison of Amitriptyline's Effects
| Condition / Symptom | Amitriptyline's Effect | Mechanism of Action |
|---|---|---|
| Constipation | Often causes or worsens | Anticholinergic effect slows gut contractions and intestinal transit time [1.4.3]. |
| Diarrhea | Can occur, but is less common | May be an initial side effect or a symptom of a condition like serotonin syndrome [1.3.2, 1.3.4]. |
| IBS with Diarrhea (IBS-D) | Often improves symptoms | Slows colonic transit time, which helps reduce the frequency of diarrhea and firms stool [1.4.5, 1.7.2]. |
| IBS with Constipation (IBS-C) | Generally worsens symptoms | The medication's primary constipating effect can exacerbate existing constipation [1.4.1]. |
Managing Bowel Changes on Amitriptyline
If you experience changes in your bowel movements while taking amitriptyline, there are several strategies you can employ. These side effects are often mild and may lessen as your body adjusts to the medication over a few weeks [1.4.6].
For Constipation:
- Increase Fiber Intake: Add more high-fiber foods to your diet, such as fruits, vegetables, and whole grains [1.5.1].
- Stay Hydrated: Drink plenty of water throughout the day to help soften stool [1.5.1].
- Exercise Regularly: Physical activity can help stimulate bowel contractions [1.5.1].
- Talk to a Professional: If lifestyle changes aren't enough, speak with your doctor or pharmacist. They may suggest an over-the-counter stool softener or laxative [1.2.2, 1.5.2]. Your doctor might also consider adjusting your dose [1.5.2].
For Diarrhea:
- Consult Your Doctor: Since diarrhea can sometimes be a sign of a more serious issue, it is important to discuss it with your healthcare provider, especially if it's severe or accompanied by other symptoms like fever or muscle stiffness [1.3.4].
Conclusion
Amitriptyline frequently changes bowel movements as a primary side effect of its pharmacological action. Its strong anticholinergic properties are very likely to cause constipation by slowing down the entire digestive process [1.4.3]. While diarrhea is also a possible, though less common, outcome, the constipating effect is so pronounced that low-dose amitriptyline is now recognized as an effective second-line treatment for managing symptoms in people with diarrhea-predominant IBS [1.6.5]. Anyone taking this medication should be aware of these potential digestive changes and discuss any bothersome or severe symptoms with their healthcare provider to find the best management strategy.
For more information on amitriptyline, you can visit the NHS page on the medication.
