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Key takeaways
While phentermine and Mounjaro can sometimes be taken together, medical experts do not generally recommend this.
Taking phentermine and Mounjaro together can result in quicker weight loss. However, this may not result in sustained health benefits.
There is a risk of experiencing negative drug interactions when taking phentermine and Mounjaro together.
You should consult your healthcare provider before taking any prescription medications to determine the best weight loss plan for you.
Phentermine, often known by the brand names Lomaira or Adipex-P, is a prescription diet pill that treats obesity by suppressing appetite. It is a stimulant medication that has become less popular for weight loss in recent years, having been surpassed by medications like Mounjaro (tirzepatide). Mounjaro is a prescription GLP-1 medication used to regulate and improve glucose levels in people with Type 2 diabetes. However, GLP-1 receptor agonists like Mounjaro also lead to weight loss. While some people may wish to take phentermine and Mounjaro together, it is not generally recommended by healthcare providers as it poses unnecessary risks to your health.
Can you take phentermine and Mounjaro together?
There are a variety of reasons why you might want to lose weight—to lower your risk of disease, to be able to live a more active lifestyle, or for increased levels of self-confidence. Often, when someone decides to lose weight—particularly, in cases of obesity, a large amount of body weight—they want to do so as quickly as possible. This may lead people to wish to take multiple drugs that induce weight loss simultaneously. But is it safe to take phentermine and Mounjaro together? Healthcare providers say that it can be—but there are risks, and it is not usually recommended.
When it comes to combining weight loss drugs, Ralph Waldo, MD, a physician at HolistiqMD in Indianapolis, Indiana, says that doing so can bring more harm than good. “Phentermine is an appetite suppressant that can cause side effects like insomnia and increased heart rate,” he says. “Mounjaro, a diabetes drug, is not approved for weight loss and may interact dangerously with phentermine.” While the U.S. Food and Drug Administration (FDA) has approved Zepbound for weight loss, which features the same active ingredient as Mounjaro, Mounjaro is only approved to help improve blood sugar in adults with type 2 diabetes—not for weight loss.
This may sound familiar to those who have sought out a prescription for Ozempic (semaglutide), which has become trendy for weight loss in recent years. Ozempic, like Mounjaro, is a GLP-1 agonist that aims to lower blood sugar. It is not FDA-approved as a weight loss medication despite often being used as such off-label due to showing efficacy in clinical trials.
If you’re taking Mounjaro and phentermine together, you should only do so under the supervision of a healthcare professional to ensure proper dosage and to be educated on short-term and long-term risks.
Benefits of taking phentermine and Mounjaro together
People who take phentermine and Mounjaro together do so to lose weight more quickly than they would taking one drug. According to David Nazarian, MD, a weight loss specialist at My Concierge MD in Los Angeles, California, this increase in weight loss speed is due to the two drugs using different pathways to suppress your appetite. “If someone is not having the desired appetite suppression from Mounjaro at higher dosages, phentermine can be added as a second weight loss medication,” he says. “I have had a few patients that this was the case for, and the two [drugs] in combination had a higher appetite suppression.”
Risks of combining phentermine and Mounjaro
While it is possible to safely combine phentermine and Mounjaro, it is not recommended as the combination increases the risk of side effects.
Additionally, Dr. Nazarian says that taking the two together can be dangerous in terms of weight loss. “There is more of a risk of dehydration and a decrease [of] protein and caloric intake in general,” he says. As phentermine and Mounjaro work as appetite suppressants, you may consume even less food and water when taking both, which can be risky to your nutrition and cause gastrointestinal symptoms.
As you consume less food and water than usual, it’s important to look for symptoms of low blood sugar (hypoglycemia), including weakness, shaking, increased heartbeat, anxiety, irritability, and dizziness.
Possible side effects of taking phentermine and Mounjaro together include:
- Dry mouth
- Constipation
- Diarrhea
- Nausea
- Headache
- Fast heart rate
Dr. Nazarian also notes that certain medical conditions can increase the risk of side effects and adverse reactions. People who have high blood pressure (hypertension) or cardiovascular issues may experience a fast or irregular heart rate and should consult with a healthcare provider before taking phentermine or Mounjaro.
Additionally, Dr. Nazarian shares that people taking Mounjaro should be careful about which other medications they’re taking, as certain medications can dangerously interact with it. Mounjaro should not be taken with other diabetes medications, such as insulin and sulfonylureas, as they can plummet blood sugar levels dangerously low when taken together. Medications that slow the GI tract should also be avoided, such as opioids and anticholinergic medications, as they can exacerbate GI symptoms such as nausea, vomiting, and constipation.
Healthcare providers will also watch which other medications you’re taking alongside phentermine, as certain prescription medications can lessen the effectiveness of phentermine. “Antidepressants and anti-anxiety medications such as selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs) can make phentermine less effective, as well as antihistamines and beta blockers,” Dr. Nazarian says.
Be sure to discuss any medications that you’re taking with your healthcare provider to ensure that you are not at risk for dangerous drug interactions and are not taking any medications that may lessen the effectiveness of other necessary medications.
Which one is better for weight loss?
Of the two drugs, Dr. Nazarian has found Mounjaro to be more effective in weight loss, as it can generally be tolerated better and used for longer periods, whereas phentermine is typically used short term. “Patients who take phentermine for longer periods tend to form a tolerance to the effects of the medication,” he says. Phentermine also has more associated side effects, making Mounjaro a more tolerable drug for consistent use.
If you’re experiencing severe or extreme side effects after taking medication, please seek medical advice immediately.
Common side effects of phentermine include:
- Dry mouth or bad taste in the mouth
- Nervousness and jitters
- Increased heart rate and blood pressure
- Tingling in the hands or feet
- Constipation
- Change in sex drive or erections
- Difficulty sleeping
- Dizziness
- Headache
- Diarrhea
- Stomach pain
- Mood changes
- Swelling of the feet and lower legs
- Rash
Serious side effects may include serious mental changes or seeing or hearing things that aren’t there.
The most common side effects of Mounjaro are related to the gastrointestinal system, which can include:
- Lack of appetite
- Nausea
- Diarrhea
- Vomiting
- Stomach pain
- Constipation
- Gas
- Fast heart rate
- Injection site reactions
In rare cases, Mounjaro can cause severe side effects, such as:
- Serious allergic reactions
- Pancreatitis (inflamed pancreas)
- Changes in vision
- Hypoglycemia (low blood sugar)
- Severe stomach and intestine problems
- Severe gallbladder problems
- Risk of thyroid cancer
- Acute kidney injury
Dr. Waldo emphasizes that the best, most sustainable weight loss results are seen through consistent lifestyle changes, not through drugs. “Quick fixes tend to be short-lived,” he says, emphasizing the importance of lifestyle intervention through nutrition and exercise.
The bottom line
Medical experts say taking phentermine and Mounjaro together may be okay. However, the risks often outweigh the benefits, and you may achieve better results with fewer side effects under a different treatment protocol that emphasizes healthy lifestyle changes alongside a single prescription drug.
Taking phentermine and Mounjaro together may achieve faster weight loss results than only taking one drug individually. Still, you may experience significant side effects and risky drug interactions, in addition to a dangerously suppressed appetite.
If you wish to lose weight, please consult your healthcare provider to determine the safest and most productive weight-loss strategy.
- FDA Approves New Medication for Chronic Weight Management, U.S. Food & Drug Administration (2023)
- About Low Blood Sugar (Hypoglycemia), Centers for Disease Control and Prevention (n.d.)
- Phentermine (Oral Route), Mayo Clinic (2024)