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Does Phentermine Cause Constipation?

Yes, phentermine can cause constipation. However, it is not quite as prevalent as some sources on the internet would have you believe. Many sources claim that nearly 50% of phentermine users experience constipation, yet we can find no evidence to support this claim. 

In fact, a study published in the Yonsei Medical Journal found that only 13% of users experienced constipation, compared to the placebo group at 8%. However, the study did find that 55% of individuals suffered from dry mouth. This was the most prevalent side effect. 

Yet another, even larger study consisting of over 700 participants showed a  lower rate of constipation at 2.3%, with the highest reported side effect being dry mouth (11%).

So, although phentermine can cause constipation, the likelihood of you experiencing this side effect is relatively low. 


A visual representation of lower gi discomfort

Why Does Phentermine Cause Constipation

There are two main reasons why you may be experiencing constipation while taking phentermine, and they are both related to diet. 

Because patients’ food intake falls as they start to take phentermine, they may start to eat fewer foods containing fiber. A diet that is rich in fiber allows your stool to absorb more water and in turn, be easier to pass. A lack of fiber may cause you to become constipated. 

The second reason is related to the first, a lack of water. As phentermine is a CNS stimulant, it can make it easier to ignore base-level needs such as hunger and thirst. For the same reason that phentermine can cause you to lose weight, it can also cause you to become dehydrated. 

Since water is extremely important for the digestive system, a lack of it can cause your stool to become hard and difficult to pass. In fact, dehydration is the most common cause of chronic constipation.


A Smiling Older Man

How To Avoid Constipation When Taking Phentermine 

To avoid constipation while taking phentermine, you can take several steps to maintain a healthy digestive system. Here are some strategies:


Increase Fiber Intake

Include a variety of high-fiber foods in your diet. This includes fruits, vegetables, whole grains, legumes, and nuts. Aim for at least 25-30 grams of fiber per day. Gradually increase your fiber intake to prevent digestive discomfort.


Stay Hydrated

Drink an adequate amount of water throughout the day. Aim for at least 8 cups (64 ounces) of water daily. Proper hydration helps soften the stool and prevents dehydration, which can contribute to constipation.

An easy way to increase water intake is to carry a big water bottle with you all day. Simply having water on hand will cause you to stay hydrated. Additionally, some water bottles come with demarcations telling you how much water you should’ve drank based on the time of day!


Exercise Regularly

Engage in regular physical activity, as exercise helps stimulate bowel movements and promotes a healthy digestive system. Aim for at least 30 minutes of moderate-intensity exercise most days of the week.


Establish Regular Meal Times

Stick to a consistent meal schedule to regulate your digestive system. Regular meal times can help stimulate bowel movements.

By sticking to a consistent meal schedule, you can train your body to anticipate and prepare for meals, optimizing the digestive process. For example, you can set fixed meal times such as breakfast at 8:00 AM, lunch at 12:30 PM, a snack at 3:30 PM, and dinner at 6:30 PM. 

Additionally, creating a meal plan in advance ensures you have the necessary ingredients and minimizes unhealthy food choices


Avoid Processed Foods

Minimize the consumption of processed foods, which are often low in fiber and high in unhealthy fats. Moreover, certain additives and preservatives commonly found in processed foods can affect digestive health and potentially contribute to constipation in some individuals. 

For example, artificial sweeteners like sorbitol and maltitol, which are commonly used in sugar-free processed foods, can have a laxative effect in high amounts. However, consuming them in moderate quantities may cause constipation or digestive discomfort

Opt for whole, unprocessed foods whenever possible.


Consider Fiber Supplements

If it’s challenging to meet your fiber requirements through diet alone, you can discuss with your healthcare provider the option of taking a fiber supplement. Psyllium husk and methylcellulose are common fiber supplements that can help relieve constipation.



While constipation is a side effect of phentermine, it’s not quite as prevalent as some sources claim it to be. Moreover, it’s not necessarily the drug itself causing constipation but instead your diet and water intake while taking phentermine. 

The best way to avoid constipation while taking phentermine is to increase your fiber and water intake. If this does not work for you, consider reaching out to a medical professional. 

If you are interested in starting your weight loss journey, reach out to Weight Wise Weight Loss Center. Our team of trained weight loss experts is here to help you every step of the way, from diet plans to medical weight loss drugs. 

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