Side Effects

GLP-1 and Constipation: Why It Happens and How to Relieve It

GLP-1 Companion · 7 min read

Constipation is among the most common gastrointestinal side effects of GLP-1 receptor agonists, affecting an estimated 15–25% of patients. Unlike nausea, which tends to peak early and improve, constipation can be persistent and may worsen as doses increase. The good news is that it responds well to a structured, stepwise management approach that most patients can implement at home. Left unaddressed, however, constipation can cause significant discomfort, reduce medication adherence, and occasionally lead to complications such as fecal impaction.

Why GLP-1 Medications Cause Constipation

The primary driver is slowed colonic transit — the rate at which waste moves through the large intestine. GLP-1 receptors are expressed in the colon as well as the stomach, and pharmacological activation slows smooth muscle contractions throughout the gastrointestinal tract. The result is that stool spends more time in the colon, allowing more water to be absorbed from it. Harder, drier stools are more difficult to pass and lead to less frequent defecation.

A secondary but important contributor is reduced food and fluid intake. GLP-1 therapy is specifically designed to reduce appetite — and eating less means less dietary bulk reaching the colon to stimulate peristalsis. Patients who are not consciously prioritizing fluid intake often become mildly dehydrated, which further concentrates the stool. This combination of slowed colonic transit, reduced dietary bulk, and relative dehydration creates the perfect conditions for constipation.

Semaglutide vs. Tirzepatide: Which Causes More Constipation?

Clinical trial data consistently shows that semaglutide (Ozempic, Wegovy) causes constipation at higher rates than tirzepatide (Mounjaro, Zepbound). In the STEP trials for semaglutide, constipation was reported in approximately 24% of patients at the 2.4 mg dose. In the SURMOUNT-1 trial for tirzepatide, constipation rates were lower, in the 11–17% range. This difference likely reflects the dual GIP/GLP-1 mechanism of tirzepatide, which may produce a more balanced effect on gut motility compared to selective GLP-1 agonism.

The Management Ladder: Step-by-Step

Rather than jumping immediately to laxatives, the evidence-based approach to GLP-1-related constipation follows a progressive ladder. Work through these steps sequentially, adding the next intervention only if the previous one is insufficient:

1. Step 1 — Increase water intake to at least 80 oz (approximately 2.4 liters) per day. This is the single most impactful first step. On GLP-1 therapy, total fluid intake frequently drops due to reduced appetite. Deliberate, consistent hydration can resolve mild constipation within 48–72 hours.
2. Step 2 — Gradually increase dietary fiber to 25–35 grams per day. Fiber is the primary bulking agent that stimulates colonic peristalsis. Increase fiber slowly (by 3–5 grams per week) to avoid bloating and gas. Good sources: oats, flaxseed, berries, cooked vegetables, whole grains.
3. Step 3 — Walk for at least 20–30 minutes daily. Physical activity directly stimulates gut motility through neurological and mechanical mechanisms. Even a brisk 20-minute walk after dinner consistently improves transit time.
4. Step 4 — Add psyllium husk supplement (Metamucil). Take 1–2 teaspoons in a full 8-oz glass of water once or twice daily. Psyllium is a soluble fiber that absorbs water and forms a gel, softening stool and stimulating movement. Always take with adequate water or it can worsen constipation.
5. Step 5 — Introduce polyethylene glycol (PEG, sold as MiraLAX). PEG is an osmotic laxative and is the safest over-the-counter laxative for regular or extended use. It works by drawing water into the stool, making it softer and easier to pass. One capful (17 grams) in 8 oz of water, once daily, is the standard dose. Unlike stimulant laxatives, PEG does not cause dependency or tolerance.
6. Step 6 — If constipation persists despite steps 1–5, discuss with your provider. They may recommend prescription options, evaluate for contributing factors (thyroid function, medications), or adjust your GLP-1 dose escalation schedule.

Why Stimulant Laxatives Should Be Avoided Long-Term

Stimulant laxatives — such as senna (Ex-Lax, Senokot) and bisacodyl (Dulcolax) — work by directly stimulating bowel muscle contractions. While effective for short-term or occasional use, regular reliance on stimulant laxatives can cause the colon to become dependent on the external stimulus, reducing its intrinsic ability to contract. This so-called "cathartic colon" effect worsens constipation over time. Stimulant laxatives also cause more cramping and urgency than osmotic agents. They are appropriate for occasional use (no more than 1–2 times per week) during episodes of acute constipation, but not as a daily management strategy.

The Role of Magnesium

Magnesium citrate and magnesium oxide work as osmotic agents in the intestine, drawing water into the bowel and stimulating movement. Magnesium citrate (as a liquid formulation) is stronger and better suited for more significant constipation episodes. Magnesium glycinate or magnesium oxide supplements taken at bedtime (200–400 mg daily) can serve as a gentle daily aid. Importantly, magnesium also has the secondary benefit of reducing muscle cramps and supporting sleep — both of which can be relevant for GLP-1 patients. Patients with kidney disease should consult their provider before using magnesium supplements.

Toilet Posture and the Squatty Potty

The standard seated toilet position creates a 90-degree angle at the anorectal junction, which actually requires muscular effort to defecate against. Elevating the feet on a small stool (7–9 inches) — as popularized by the "Squatty Potty" — creates a more natural squatting position that straightens the anorectal angle and makes defecation significantly easier with less straining. For patients experiencing constipation on GLP-1 therapy, this simple, inexpensive intervention can meaningfully reduce straining and improve evacuation.

Red Flags: When to Contact Your Provider

While GLP-1-related constipation is usually manageable at home, the following situations warrant medical evaluation:

• No bowel movement for 7 or more days despite using the interventions above
• Severe abdominal pain, cramping, or distension
• Nausea and vomiting accompanying constipation — raises concern for bowel obstruction
• Blood in the stool or on toilet paper
• Alternating severe constipation and explosive diarrhea — may indicate an underlying motility disorder
• Constipation developing in a patient with no prior history who recently started GLP-1 therapy alongside other new medications

Does Constipation Improve Over Time?

Unlike some GLP-1 side effects that clearly improve with adaptation, constipation can persist throughout therapy — particularly if dietary fiber and fluid intake remain insufficient. The mechanism (slowed colonic transit) does not fully resolve with dose stability the way nausea does. This means proactive, ongoing dietary management is the key to long-term bowel health on GLP-1 therapy. Patients who build adequate hydration, fiber intake, and daily walking into their routine consistently report good bowel health throughout their treatment course.

Sources

• Drucker DJ (2018). Mechanisms of Action and Therapeutic Application of GLP-1. Cell Metabolism.
• Ozempic (semaglutide) Prescribing Information. Novo Nordisk.