Managing Constipation While Using Injectable Weight Loss Medications (Semaglutide or Tirzepatide)

Injectable weight-loss medications such as semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) have transformed obesity treatment by helping people reduce appetite, improve blood sugar control, and achieve meaningful weight loss. However, one of the most common side effects people experience when starting these medications is constipation.

There are several scientific reasons why constipation can occur while using GLP-1 or dual GLP-1/GIP medications. First, these medications slow gastric emptying and intestinal motility, meaning food moves through the digestive tract more slowly. This effect helps people feel full longer, but it can also lead to harder or less frequent bowel movements. Second, many people eat significantly less food overall, which means there is simply less stool volume moving through the colon. Finally, reduced food intake may also lead to lower fibre and fluid intake, both of which are essential for maintaining healthy bowel movements.

The good news is that constipation is usually most noticeable during the dose escalation (titration) phase when the medication is gradually increased. Once people reach their maintenance dose, the digestive system often adapts and symptoms become more tolerable or resolve entirely.

As a dietitian working in obesity medicine, I’ve helped many patients manage constipation while using GLP-1 medications, and most people are able to improve their symptoms with simple, practical strategies. Below is a step-by-step approach that I commonly recommend.

1. Prioritize Hydration

Adequate hydration is the foundation of constipation management.

Water helps keep stool soft and easier to pass. When food moves more slowly through the digestive tract, the colon absorbs more water from stool, which can make it harder and more difficult to pass.

A general target for most adults is about 2 liters of fluids per day, though individual needs vary depending on body size, activity level, and climate.

Practical hydration tips:

• Sip water consistently throughout the day rather than drinking large amounts all at once

• Include water-rich foods such as fruits, vegetables, and soups

• Consider drinking a glass of water with meals and snacks

Many people taking GLP-1 medications drink less simply because they feel less hungry or thirsty, so hydration often needs to be intentional.

2. Gradually Increase Fibre Intake

Fibre helps add bulk and softness to stool, making bowel movements easier. However, when increasing fibre, slow and gradual changes are key.

A helpful strategy is to increase fibre intake by about 3 grams per day, working toward a total intake of approximately 20–25 grams daily.

For people experiencing constipation on GLP-1 medications, soluble fibre tends to be especially helpful.

Why soluble fibre?

Soluble fibre dissolves in water to form a gel-like substance that helps soften stool and improve stool consistency. It is often better tolerated than large amounts of insoluble fibre, which can sometimes worsen bloating.

Good sources of soluble fibre include:

• Oats and oat bran

• Chia seeds

• Ground flaxseed

• Psyllium husk

• Beans and lentils

• Apples and pears

• Barley

If fibre intake increases too quickly, it may cause gas or bloating, so gradual increases allow the digestive system to adjust.

3. Move After Meals

Gentle physical activity after eating can help stimulate bowel movements.

Movement activates what is known as the gastrocolic reflex, a natural physiological response in which the colon becomes more active after a meal. This reflex helps move stool through the large intestine.

Even light activity can help, such as:

• A 10–15 minute walk after meals

• Light stretching

• Household movement

Timing matters. Moving shortly after eating helps take advantage of this natural reflex and can promote more regular bowel movements.

4. Consider Magnesium Supplements

Magnesium supplements can help draw water into the intestines and soften stool.

A common dose used for constipation support is around 350 mg per day, though this can vary depending on the individual.

There are several forms of magnesium:

• Magnesium oxide – commonly used for constipation and may have a stronger laxative effect

• Magnesium citrate – also effective for promoting bowel movements

• Magnesium glycinate – gentler on the stomach and often preferred for long-term use

In practice, any form of magnesium may help, but some people find certain forms cause more gastrointestinal upset than others.

5. Use Osmotic Laxatives When Needed

If lifestyle strategies are not enough, over-the-counter osmotic laxatives can be helpful.

In Canada, a commonly used option is PEG 3350 (polyethylene glycol), sold under brand names such as Restoralax or Miralax.

Osmotic laxatives work by drawing water into the stool, making it softer and easier to pass. They are generally well tolerated and can be used as needed.

6. Senna-Based Laxatives Are Safe for Long-Term Use

Another option is senna-based laxatives, such as Senokot.

These medications stimulate the muscles of the colon to help move stool through the digestive tract. Despite common myths, research has shown that senna can be safe for long-term use when needed, especially when used appropriately.

For some individuals taking GLP-1 medications, a low dose used occasionally can help maintain regular bowel movements.

The Bottom Line

Constipation is a common side effect when starting injectable weight-loss medications like semaglutide or tirzepatide. It often occurs because these medications slow the digestive system and reduce overall food intake.

Fortunately, most cases improve with simple strategies such as:

• Staying well hydrated

• Gradually increasing fibre intake

• Moving after meals

• Using magnesium supplements

• Adding osmotic laxatives when needed

Symptoms are typically most noticeable during the titration phase and often improve once the body adjusts to the medication.

If constipation becomes severe or persistent, it’s important to discuss it with your healthcare provider so your treatment plan can be adjusted.

About Evan Huang-Ku, RD, MPH

Evan Huang-Ku, RD, MPH is a Registered Dietitian based in Canada who works in obesity medicine and metabolic health. He provides nutrition counseling to individuals using lifestyle strategies, medications, and other treatments to support sustainable weight management.

Through his clinical work, Evan has helped many patients navigate the practical challenges of weight-loss medications, including managing gastrointestinal side effects such as constipation.

His approach focuses on evidence-based nutrition, realistic lifestyle changes, and long-term sustainability.