Melatonin is a possible cause of constipation, but the side effect is uncommon. Constipation appears among the less common digestive side effects of melatonin. Research reviews describe gastrointestinal reactions as infrequent and mild, and they do not consistently occur more often with melatonin than with a placebo.
A 2019 systematic review reported constipation among the digestive reactions linked to oral melatonin, but the evidence did not show that melatonin commonly causes constipation. Adults generally need about 22 to 34 grams of fiber per day, depending on age and sex, and low fiber intake is one possible cause of constipation.
Melatonin and Constipation at a Glance
| Question | Answer |
|---|---|
| Can melatonin cause constipation? | Yes, but it is uncommon. |
| Is constipation a typical melatonin side effect? | No. Sleepiness, headache, dizziness and nausea are reported more often. |
| Should you stop melatonin? | Speak with a pharmacist or clinician, especially if melatonin was prescribed. |
| What else may be causing constipation? | Dehydration, low fiber intake, reduced activity, travel, or other medicines and supplements. |
| When is constipation urgent? | Seek prompt medical care for severe or constant abdominal pain, vomiting, blood in the stool, fever, or an inability to pass gas. |
Why Might Melatonin Cause Constipation?
The exact reason is not known. Melatonin affects the sleep and wake cycle, and it may also affect gastrointestinal activity. Available evidence does not show a strong or consistent constipation effect in most people.
The 2019 systematic review found that reported side effects were usually minor and short-lived. Constipation was among the digestive reactions reported, but the evidence was not strong enough to show that melatonin commonly causes it.
Constipation may also begin around the same time as melatonin use for another reason, such as:
- Not drinking enough fluids
- Eating less fiber
- Getting less physical activity
- Travelling or changing your usual routine
- Taking iron or calcium supplements
- Using opioid pain medicines
- Taking some antidepressants, antacids or other medicines
Because these factors can also cause constipation, the timing of the symptom does not prove that melatonin is responsible.
How to Tell Whether Melatonin May Be Contributing
Melatonin may be contributing if:
- Constipation began soon after you started taking it.
- The symptom improves after a clinician recommends lowering the dose or stopping melatonin.
- Constipation returns when melatonin is restarted.
- No other medicine, supplement, dietary change or lifestyle change explains the symptom.
Do not stop a prescribed medicine without medical advice. If you take over-the-counter melatonin, a pharmacist can review the dose, product ingredients and other medicines that may be contributing.
What to Do if Melatonin Makes You Constipated
For mild constipation, these general measures may help:
- Drink enough fluids unless a clinician has restricted your intake.
- Add fiber gradually through foods such as oats, beans, vegetables, fruit and whole grains.
- Stay physically active.
- Go to the bathroom when you feel the urge.
- Ask a pharmacist whether a short-term laxative is suitable.
Adults generally need about 22 to 34 grams of fiber per day, depending on age and sex. Add fiber slowly and drink enough liquid. Increasing fiber without enough fluid can worsen bloating or discomfort.
Is Constipation More Likely With Some Melatonin Products?
Possibly. Melatonin supplements differ in dose, formulation and added ingredients. Gummies, tablets and combination sleep products may contain other substances that affect digestion.
Check the full ingredient list, not just the amount of melatonin. Some products also contain minerals, herbs or sweeteners. A pharmacist can help you work out whether the symptom is more likely related to melatonin, another ingredient or a separate medicine.
When Should You Seek Medical Care?
Contact a healthcare professional if constipation does not improve with self-care or keeps coming back.
Seek urgent medical attention if constipation occurs with:
- Constant or severe abdominal pain
- Vomiting
- Blood in the stool or rectal bleeding
- Fever
- Unintentional weight loss
- An inability to pass gas or stool
- A swollen, hard or very tender abdomen
These symptoms can point to a problem that needs more than routine constipation treatment.
Bottom Line
If constipation started after you began taking melatonin, review the timing, dose, product ingredients, fluid intake and other medicines with a pharmacist or healthcare professional. Constipation is possible, but it is not one of melatonin's main expected side effects.