Does phentermine cause constipation?

In this article:

• Does phentermine cause constipation?
• Phentermine and constipation
• Treatment
• When to see your healthcare provider

In this article

• Does phentermine cause constipation?
• Phentermine and constipation
• Treatment
• When to see your healthcare provider

Just under half of U.S. adults attempt to lose weight in any given year, according to the CDC. Sometimes it’s with diet and exercise. Other times, with prescription medicine. And among all the weight loss drugs on the market, phentermine has been the most widely used since its FDA approval in 1956. From a class of medications called anorectics, it can help suppress the appetite to assist weight loss. This drug may have helped millions of Americans shed unwanted pounds, but it also comes with the risk of common side effects, including constipation. 

It doesn’t happen to everyone, but it’s possible if you take phentermine for weight loss. How does it cause constipation? How can you manage it? When should you see a healthcare provider? Read on for all the health information you need. 

Does phentermine cause constipation?

Yes, constipation is one of the most common side effects of phentermine. According to a study from 2021, 3.5% of patients who took 15 mg for six months reported it, along with 4.1% of patients who took 30 mg. 

Phentermine is often combined with an anticonvulsant called topiramate. The duo works well together to reduce appetite, but may also cause constipation, along with other potential side effects. In clinical trials for a combination of phentermine and topiramate called Qsymia, 16% of patients taking 15 mg phentermine/92 mg topiramate, 15% of patients taking 7.5 mg phentermine/46 mg topiramate, and 8% of patients taking 3.75 mg phentermine/23 mg topiramate reported constipation—compared to 6% of patients in the placebo group. 

However, some people experience the opposite, since diarrhea is also one of phentermine’s potential adverse effects. It depends on the specific person and their body’s reaction to the drug.  

Not all weight loss pills and appetite-suppressant drugs cause constipation, but because they often affect the digestive tract, many of these medications can cause constipation or other gastrointestinal side effects like nausea or stomach cramps. It’s also one of the most prevalent adverse reactions for GLP-1 medications like Zepbound, Wegovy, and Saxenda, as well as other weight-loss medications like Contrave. 

Relationship between phentermine and constipation

Phentermine can cause constipation because of the way it works within the body. According to Dr. Pratima Dibba, MD, board-certified gastroenterologist with Medical Offices of Manhattan and contributor to https://www.labfinder.com: “Phentermine works on a certain part of the nervous system and increases a chemical messenger, also known as a neurotransmitter, called norepinephrine, which can impact the gastrointestinal tract, slowing motility and gastric secretion.”

Essentially, the drug activates the nervous system’s “fight or flight” response, which diverts energy away from digestion in favor of what it presumes to be more urgent functions. As part of this process, it slows down peristalsis—the series of muscle contractions that push food through the digestive tract. This slowed digestion helps with weight management, but it also might delay bowel movements. 

“It can also cause dry mouth, which makes it easier for your body to lose water. It also speeds up your metabolism, which can make you sweat more, especially if you’re active,” says Dr. David Nazarian, MD, founder of My Concierge MD. “Another thing is that phentermine can sometimes make it harder for your body to tell when you’re thirsty. This can make you less likely to drink enough water, especially when you’re sweating or losing fluids in other ways. When your body doesn’t have enough fluids, it can affect your digestive system and cause constipation.”

How long phentermine constipation lasts can depend on the person. However, the medication is generally only for short-term use. The Food and Drug Administration has only approved it for treatments of up to 12 weeks and typically stays in your system for four days after you stop taking it. 

Management and treatment of phentermine-induced constipation

It’s not ideal to get constipation from phentermine treatment, but the good news is that you’ve got ways to deal with it. Here are a few of the most effective. 

• Eat fiber: As we mentioned, fiber is a crucial component of the digestive process (and can help regulate blood sugar), so make sure you’re eating enough—the USDA recommends 28 grams per day for most people. Good sources of fiber include fruits, vegetables, legumes, whole grains, and nuts. Dietary supplements may also help, if you find it hard to hit the recommended amount. 

• Drink plenty of fluids: This is good advice for everyone, but especially anyone taking phentermine, since dehydrated stool is harder to pass. According to the U.S. National Academies of Sciences, Engineering, and Medicine, men typically need at least 15.5 cups of fluids per day, and women need at least 11.5. However, this also depends on your weight, activity level, environment, and age. 

• Avoid certain foods: Processed foods, fried foods, dairy, eggs, meat, and alcoholic beverages can all exacerbate constipation, so consuming them in moderation may help. 

• Exercise regularly: Physical activity can help stimulate the digestive system and move food more quickly through the large intestine, and studies have shown that it’s an effective treatment for constipation. 

• Try bowel training: This means trying to have a bowel movement at the same time every day, which can get your body into a routine. Choose a convenient time to sit on the toilet each day, and be patient, since it may take 15–45 minutes. Sometimes, a gentle abdominal massage can help. 

What medications can I take for constipation?

Dietary and lifestyle changes are great for constipation (and your overall health), but sometimes, your gut needs a little more encouragement. That’s where medications can help. Here are some laxative options that might help. 

• Bulk-forming laxatives: Drugs like Metamucil or Citrucel add soluble fiber to your stool, which helps it pull in water, making it bigger and easier to pass. These often pose the smallest risk of serious side effects, so they’re commonly the first ones health care providers might recommend. 

• Osmotic agents: These medicines take water from different parts of the body and direct it to the colon, where it softens the stool. Examples of these medications include Miralax and Milk of Magnesia.

• Emollient laxatives: Commonly called stool softeners, drugs like Colace help the stool collect and hold more moisture, making it easier to pass. 

• Lubricant laxatives: As the name suggests, these laxatives lubricate the colon, making it easier for stool to pass through (an example is mineral oil). 
• Stimulant laxatives: Drugs like Dulcolax trigger muscle contractions around the intestinal walls, which helps push the stool along. However, they’re typically only recommended if other laxatives have failed, since some individuals who take them start relying on them for every bowel movement, leading to long-term use. 

The best option for constipation from phentermine depends on you and your specific circumstances. It’s best to ask a doctor for their medical advice since they’re familiar with your medical history, other medicines, and overall health. Sometimes, physicians suggest prescription drugs like lactulose or Linzess, but that’s usually in cases of chronic constipation—not temporary constipation that comes as a side effect. 

When to see your healthcare provider

Most of the time, the biggest risk of constipation from phentermine is physical discomfort. It can make you feel full, bloated, nauseous, and frustrated. Aside from that, it’s usually nothing to be alarmed about. 

But in rare cases, constipation may indicate a more serious condition, or require medical attention. For example, “it is important to speak to a physician when symptoms are refractory to a high fiber diet,” Dr. Dibba says. So if you’ve loaded up on fiber and your constipation persists, call a doctor. 

Additionally, according to Dr. Nazarian, “If constipation is severe, lasts for a long time, or is accompanied by other symptoms like blood in your stool, pain, nausea, or vomiting, it’s important to consult with a doctor so we can help determine the cause and recommend the best treatment.”

One of phentermine’s other risk factors is that it can cause high blood pressure and worsen heart disease. So seek medical help if you experience symptoms like a fast heart rate, irregular heartbeat, chest pain, shortness of breath, or fainting.