Phentermine and Constipation

Phentermine and Constipation

This article is for informational purposes only and should not be used as a substitute for professional medical advice. Always consult your healthcare provider before starting any new medication or supplement regimen.

Too busy for the full read? Here’s what you need to know:

  • Phentermine is an appetite suppressant that can lead to reduced food intake and decreased fiber consumption. This may result in constipation as a side effect.
  • To help prevent constipation, those taking phentermine should focus on staying well hydrated by drinking plenty of fluids daily.
  • Eating a fiber-rich diet with fruits, vegetables, whole grains, beans, nuts, and seeds can also help maintain bowel regularity. Getting regular exercise supports healthy digestion too.
  • If constipation occurs, adjusting the phentermine dosage under medical supervision may be warranted. The use of laxatives or stool softeners may also be recommended by one’s doctor to relieve symptoms.

Phentermine, a commonly prescribed weight loss medication to suppress appetite, can have the unwanted side effect of constipation. In this article, we will explore the causes of phentermine constipation and provide practical remedies to alleviate this discomfort.

What Is Constipation?

Constipation refers to infrequent bowel movements or difficulty passing stools. It typically involves hard and dry stools that are painful or require straining to pass.

Common symptoms include fewer than three bowel movements per week, lumpy or hard stools, straining during bowel movements, the sensation of incomplete evacuation after a bowel movement, and abdominal pain or discomfort ( 1 , 3 ).

There are various causes of constipation. Dehydration can lead to hardening of stools due to lack of fluids. Inadequate fiber intake also contributes as fiber helps add bulk and moisture to stools. Some medications, such as phentermine, can cause constipation as a side effect ( 1 ).

Does Phentermine Cause Constipation?

woman with phentermine constipation
Nearly half of phentermine patients experience phentermine constipation

Yes, constipation is one of the most common side effects of phentermine.

In fact, it is the third most common side effect of this medication, and almost 50% of users report phentermine constipation ( 2 ).

In addition to the day-to-day discomfort and pain, ongoing phentermine constipation can also slow weight loss.

Thankfully, most cases of phentermine constipation resolve within a few days without medical intervention.

However, if your bowel movements are irregular, infrequent, or hard to pass for more than a couple of days while taking phentermine, contact your prescribing doctor.

Why Does Phentermine Cause Constipation?

Phentermine is known to potentially cause constipation due to its effects of slowing digestion. The slow transit of food through the digestive system results in excessive water absorption from the stool. This leads to hard, dry stools that are difficult to pass.

Let’s look at the causes in greater detail:

CNS Stimulation

Drink enough water
Exercise and hydration often function as at-home phentermine constipation remedies

Phentermine stimulates the central nervous system. This causes a release of neurotransmitters that increase energy and suppress appetite. Unfortunately, these same chemicals can also cause unwanted side effects like constipation.

As part of the “fight or flight” response that produces a reduction in appetite, the brain also signals the body to divert resources away from active digestion, which slows peristalsis: muscle contractions that move food through the digestive tract ( 4 ). When digestion is slowed for too long, it can cause constipation. This is why many phentermine users complain of problems passing stool.

Inadequate fluid intake may also contribute to phentermine constipation, in addition to exacerbating dry mouth and headaches. Stimulants can cause dehydration, and if there is not enough water in the body available to help expel waste products through stools, which are typically 75% water, it may cause constipation ( 5 , 6 ).

Dietary & Emotional Changes

stressed woman with phentermine constipation
Stress can affect bowel movements and may worsen phentermine constipation

Constipation can also be a result of changes in routine, stress, depression, or anxiety.

This medication is a strong appetite suppressant, so people tend to eat significantly less while taking it. Decreased food intake can slow digestion and produce this uncomfortable side effect ( 7 ).

Phentermine constipation is especially common in patients whose diets limit the intake of certain food groups. For example, low-carb diets (like carnivore and keto) pose a particular threat, given followers’ tendency to dramatically decrease the intake of fiber-rich foods like whole grains and produce.

Stress, depression, and anxiety can also cause or worsen constipation ( 3 ). Major weight loss is inherently stressful, and phentermine can cause depression, anxiety, and other mood changes. As a result, for many users, emotional distress may also contribute to phentermine constipation.

How to Manage Phentermine-Induced Constipation

Going a day or two without a bowel movement might have you reaching for the laxatives, but consider a natural phentermine constipation remedy before you run to the drug store. Many behaviors that naturally help relieve phentermine constipation boost weight loss as well.

Here are six practical and effective ways to stop constipation from phentermine:

1. Hydration: Increase water intake

The easiest phentermine constipation remedy is to increase fluid intake.

Your organs need water to function properly, and the digestive system is no exception. When there is insufficient water, the body pulls fluid from the stool, making feces dry and hard to pass. To minimize dehydration, try to drink at least eight glasses (two liters) of water or other calorie-free, caffeine-free beverages each day. Avoid dehydrating drinks such as coffee, colas, energy drinks, and alcohol.

If you’re already drinking plenty of water, try adding a glass of warm water with lemon in the morning. Warm water increases the intestines’ tightening, facilitating bowel movements. The lemon helps as its high acid content stimulates the digestive system to get things moving ( 8 ).

broccoli, which contains fiber to relieve phentermine constipation
Eating more fiber-rich foods, like broccoli, can help relieve phentermine constipation

2. Diet: High-fiber foods

There are two types of dietary fiber, and both help to treat and prevent constipation. Soluble fiber helps decrease cholesterol levels and makes stools easier to pass, while insoluble fiber helps promote regular bowel movements ( 9 ).

Adult women need 25 grams of fiber per day, while men require 38 grams on average ( 10 ).

To fight phentermine constipation, eat more fiber-rich foods like whole fruit and vegetables with edible skins, whole grains, and legumes (e.g., lentils, beans, or peas).

A tip: Prunes, or dried plums, are rich in insoluble fiber and contain sorbitol, which is a natural laxative. In a 2018 study, subjects experienced significant increases in both stool frequency and stool weight after consuming 80-120 grams of prunes per day ( 12 ). That’s about 10-14 prunes each day.

NOTE: Eating more fiber can worsen constipation if you’re not getting enough fluids. So, as you increase fiber intake, make sure to drink plenty of water as well.

3. Physical Activity: Regular exercise

Regular physical activity increases blood flow to the intestines and promotes keeping your abdominal muscles in shape. Both help keep stool moving.

So, make exercise a part of your daily routine. Aerobic exercises like walking, running, swimming, or cycling provides the most benefit, but even a little gentle stretching or yoga can help ( 11 ).

4. Medication Adjustment

Seek guidance from your healthcare provider. This consultation could involve discussing the possibility of modifying the phentermine dosage to alleviate the discomfort.

Furthermore, healthcare professionals might suggest the temporary use of stool softeners or over-the-counter laxatives to manage symptoms as adjustments to the appetite suppressant regimen are implemented under medical supervision.

5. Stress Management

Stress, depression, and anxiety worsen constipation, and phentermine is known to exacerbate these negative emotions, so effective stress management is critical for both weight loss and phentermine constipation relief.

Reduce stress with yoga, meditation, deep breathing, doodling, reading a book, or talking to a friend.

6. OTC Medications

Several over-the-counter (OTC) remedies are available for treating different stomach issues.

You can use docusate sodium (Colace) or polyethylene glycol (Miralax) for constipation. Loperamide (Imodium A-D) can be a suitable option in case of diarrhea.

However, if your stomach problems persist even after a few days of using these OTC medications, you should inform your healthcare provider.

Can I Take a Laxative with Phentermine?

Ask a pharmacist before combining phentermine with Miralax or any other laxative

ALWAYS check with your doctor and pharmacist before taking any additional medications or supplements alongside phentermine.

Some people take a laxative with phentermine to help relieve mild, short-term constipation. However, these medications are not safe for everyone, and they act quickly, so do not use laxatives and phentermine together without talking to your doctor & pharmacist first.

Do NOT take stool softeners or laxatives for more than two weeks at a time. Consult a doctor if phentermine constipation interferes with your daily activities, worsens, or does not go away.

Back to All Phentermine Side Effects

  1. Mayo Clinic Staff. (2018, January 10). Constipation.
  2. Members of “Losing Weight with Phentermine” Support Group on Facebook & Forum. (2019, March 13). [User Report of Common Phentermine Side Effects]. Unpublished raw data
  3. National Health Service. (2017, December 20). Constipation.
  4. Silverthorn, D. U. (2009). Human physiology: An integrated approach (4th ed.). Harlow: Pearson Education.
  5. Martin, E. B., Jr., & Hammerness, P. G. (2014, August). Thirsty: ADHD, Stimulant Medication, and Dehydration. Attention, 26-27.
  6. The Editors of Encyclopaedia Britannica. (2002, June 14). Feces. In Encyclopaedia Britannica.
  7. Towers, A. L., Burgio, K. L., Locher, J. L., Merkel, I. S., Safaeian, M., & Wald, A. (1994). Constipation in the elderly: Influence of dietary, psychological, and physiological factors. Journal of the American Geriatrics Society, 42(7), 701-706. doi:10.1111/j.1532-5415.1994.tb06527
  8. Baucom, M. (2017, April 3). Do This and That: Taking Proven Chronic Constipation Remedies to the Next Level (E. K. Luo MD, Ed.).
  9. UCSF Medical Center. (n.d.). Increasing Fiber Intake.
  10. Dahl, W. J., & Stewart, M. L. (2015). Position of the Academy of Nutrition and Dietetics: Health Implications of Dietary Fiber. Journal of the Academy of Nutrition and Dietetics, 115(11), 1861-1870. doi:10.1016/j.jand.2015.09.003
  11. Smith, M. W. (Ed.). (2018, June 25). Exercise for Constipation Relief: Which Exercises to Do.
  12. Lever, E., Scott, S. M., Louis, P., Emery, P. W., & Whelan, K. (2018). The effect of prunes on stool output, gut transit time and gastrointestinal microbiota: A randomised controlled trial. Clinical Nutrition. doi:10.1016/j.clnu.2018.01.003