With more than 40% of American adults considered obese or seriously obese, it’s hardly surprising that effective weight loss solutions are in high demand. Lifestyle changes like diet planning and regular exercise are vital to any weight loss process, but medications can also expedite the journey. More than half of Americans report that they would be interested in taking a safe and effective weight loss treatment, and many are actively seeking the best weight loss medication for their needs every single day.

Phentermine (phentermine hydrochloride) and Mounjaro (tirzepatide), also sold under the brand name Zepbound, are both popular medications used by many Americans for weight loss purposes. If you are currently in search of a drug that can help you kick-start your weight loss journey or get you past a frustrating plateau, you may have already heard of them both. Like many others, you may be wondering exactly what sets these drugs apart from each other and which one may be the better fit for your particular needs and circumstances. That’s exactly why we’ve created this post.

Below, we’ve provided an overview of Phentermine and Mounjaro as well as explored their similarities and differences in terms of mechanism of action, side effects, effectiveness for weight loss, and more. Though the information provided in this post does not constitute medical advice, it can provide the background for a more in-depth conversation with your healthcare provider. Ultimately, the final decision about which medication to take (if either) should be made with support from a doctor or medical professional.

Key Differences

  • Mounjaro and Phentermine have completely different mechanisms of action: Mounjaro is classed as a dual GLP-1 and GIP receptor agonist while Phentermine is a central nervous system stimulant.
  • Mounjaro and Zepbound are long-term medications intended for the chronic management of Type 2 Diabetes and/or obesity. Phentermine is typically prescribed as a short-term weight loss treatment.
  • Mounjaro is a once-weekly injectable medication. Phentermine is taken orally by tablet once daily.
  • Mounjaro is a multi-faceted medication that targets blood glucose levels as well as body weight. Phentermine is only an appetite suppressant.


What Is Phentermine?

Phentermine (phentermine hydrochloride) is a prescription medication sold under brand names including Adipex-P, Fastin, Suprenza, and more. It’s also available in various generic forms and is often prescribed as part of a broader weight loss plan for individuals who suffer from obesity. Approved by the FDA in 1959, the drug is meant to be used as a short-term weight loss treatment, though it is sometimes prescribed off-label for longer-term use or to treat other conditions. It comes in capsules or tablets that are taken orally once daily and is meant to be combined with lifestyle changes including a balanced, calorie-deficient diet and regular exercise.

Phentermine is a stimulant medication that works by suppressing appetite. By stimulating the central nervous system, it sends chemical messages to the brain communicating that your body is full or satiated, even if this is really not the case. As a result, you eat smaller portions and easily shed pounds in a relatively short time frame (the average length of time patients take Phentermine is approximately 12 weeks total).

As a stimulant, phentermine often causes jitteriness and can also improve energy levels and mood for some patients. That being said, its structure is similar to that of amphetamines and, though it doesn’t have the same dependency or withdrawal issues, it is not recommended for patients with a history of addiction or substance abuse.

Phentermine Benefits

  • Phentermine is a well-established weight loss drug with many decades of research on its efficacy and effects.
  • It’s an oral medication (ideal for patients who are injection-averse).
  • It’s generally very affordable (available in generic form).
  • Weight loss results are usually rapid and substantial.
  • You don’t have to commit to taking the drug for a long period of time.

What Is Mounjaro

Mounjaro (tirzepatide) is a brand-name prescription medication manufactured by Eli Lilly. FDA-approved for the treatment of Type 2 Diabetes in adults since 2022, it’s also known for its notable weight loss outcomes and is often prescribed off-label for weight loss purposes. Zepbound, another tirzepatide drug identical to Mounjaro in composition, has also been approved for chronic weight management in obese and overweight individuals (with or without Type 2 Diabetes) since 2023.

A once-weekly medication, Mounjaro comes in single-use injectable pens and is injected subcutaneously (into the fatty layer below the skin). It is most effective in combination with lifestyle interventions including a balanced, calorie-deficient diet and regular exercise routine.

Mounjaro is a first-in-class dual glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptor agonist drug. It mimics two separate incretin hormones produced naturally in the human gut and binds to their receptor sites, triggering a variety of reactions in different body systems. Among other things, tirzepatide stimulates pancreatic insulin secretion in response to food, inhibits the release of sugars from the liver, slows down the speed at which food is digested in the gut, and impacts appetite centers in the brain. In turn, these mechanisms of action help regulate blood glucose levels as well as reduce hunger and cravings—often leading to significant weight loss outcomes.

Mounjaro Benefits

  • Mounjaro targets multiple body systems and functions. It helps regulate and control blood glucose levels while also promoting healthy weight loss, making it an ideal option for patients dealing with blood sugar issues in addition to being overweight or obese.
  • Conveniently, Mounjaro only needs to be taken once per week.
  • It is intended for chronic weight management and can help you maintain your weight loss goals over time.

Phentermine Dosage

Phentermine is available as oral capsules, extended-release capsules, or disintegrating tablets. It’s standard dose options include:

  • 15 mg
  • 30 mg
  • 37.5 mg

Always take phentermine exactly as prescribed by your doctor or healthcare provider.

Mounjaro Dosage

Mounjaro comes in 6 different doses. Patients typically start by taking the starting dose of 2.5 mg for four weeks before titrating up to the 5 mg dose. After that, Mounjaro’s dose may be further increased in 2.5 mg increments every four weeks (or at longer intervals, depending on your doctor’s advice) to a maximum of 15 mg. Once the body has adjusted to an effective maintenance dose of Mounjaro, the drug continues to be injected weekly over the long term.

Mounjaro comes in single-use pens with different-colored labels. Each color corresponds to a specific dose as follows:

  • Gray label – 2.5 mg
  • Purple label – 5 mg
  • Green label – 7.5 mg
  • Pink label – 10 mg
  • Blue label – 12.5 mg
  • Orange label – 15 mg

Always take Mounjaro exactly as prescribed by your doctor or healthcare provider.

How Long It Takes To Lose Weight on Phentermine

Phentermine is a short-term medication, so it’s not surprising that weight loss begins very soon after starting to take it for most patients. A quantitative analysis of data from more than four decades worth of clinical trials for obesity medications indicates that patients who take Phentermine for a maximum of 24 weeks lost an average of approximately 14 lbs. in that timeframe.

Anecdotal reports suggest that off-label prescriptions for long-term Phentermine use may result in more significant weight loss outcomes. That being said, the drug is not approved for long-term use and taking it for longer than recommended may put you at risk. Be sure to carefully discuss the potential benefits and risks associated with any off-label weight loss prescription with your doctor.

How Long It Takes To Lose Weight on Mounjaro

According to the results from a recent tirzepatide clinical trial, Mounjaro and Zepbound offer patients unprecedented weight loss outcomes. 57% of overweight and obese adults who received a weekly injection of 10 or 15 mg of tirzepatide over a 72-week period lost 20% of their starting body weight or more in combination with regular exercise and diet changes.

While every patient will react slightly differently to Mounjaro, most patients tend to experience modest weight loss almost immediately upon starting the drug followed by more significant weight loss after approximately the 12-week mark (once the body has had time to adjust to higher doses of the drug). For more information about how quickly you can expect to lose weight on Mounjaro, see our blog post dedicated specifically to that topic. **Link internally to blog post titled “How Quickly Do You Lose Weight On Mounjaro?” once published.

Phentermine and Mounjaro Side Effects

Like all medications, Phentermine and Mounjaro both come with the risk of potential side effects. We’ve outlined some of the most common and serious below. For more detailed information about side effects, see the prescribing information for Phentermine and the prescribing information for Mounjaro.

Side Effects of Phentermine

Not all patients taking Phentermine will experience side effects. If you do experience bothersome or concerning side effects while taking the drug, speak with your doctor or seek medical attention immediately.

Common Side Effects

  • Dry mouth
  • Jitteriness
  • Increased heart rate
  • Anxiety and/or irritability
  • Constipation
  • Insomnia
  • Dizziness and/or loss of alertness

Serious Side Effects

  • Serious allergic reactions
  • High blood pressure
  • Heart attack
  • Stroke
  • Primary pulmonary hypertension
  • Valve regurgitation

Side Effects of Mounjaro

Not all patients taking Mounjaro will experience side effects. If you do experience uncomfortable or concerning side effects while taking the drug, speak with your doctor or seek medical attention immediately.

Common Side Effects

  • Nausea and/or vomiting
  • Abdominal pain
  • Decreased appetite
  • Constipation
  • Diarrhea
  • Injection site reactions

Serious Side Effects

  • Serious allergic reactions
  • Severe hypoglycemia
  • Vision problems
  • Kidney, gallbladder, or pancreatic problems
  • The development of thyroid tumors

Phentermine and Mounjaro: Warnings and Precautions

In addition to the risk of side effects, Phentermine and Mounjaro both come with specific warnings and precautions, which we’ve outlined below:

Phentermine Warnings and Precautions

  • Phentermine should not be taken with alcohol.
  • It should not be taken with certain types of antidepressants as well as certain other medications (speak with your doctor for specifics).
  • Phentermine can be habit-forming.
  • It should not be taken if pregnant or breastfeeding.

Mounjaro Warnings and Precautions

  • Mounjaro should not be taken by patients with a personal or family history of Medullary Thyroid Carcinoma or Multiple Endocrine Neoplasia Syndrome Type 2.
  • It should never be taken by patients with Type 1 Diabetes or who are under 18 years of age.
  • It is not recommended for people with a history of severe digestive issues, diabetic retinopathy, or serious pancreas, kidney, or gallbladder problems.
  • Mounjaro is not approved if you are pregnant or breastfeeding.

Can I Take Mounjaro and Phentermine Together?

Taking Mounjaro and Phentermine together is not recommended. Combining weight loss medications may seem tempting, but can lead to serious outcomes. Always take medications exactly as prescribed by your doctor or healthcare provider and be sure to consult with them before starting or stopping any additional treatments.

The Verdict?

To conclude, Phentermine and Mounjaro (tirzepatide) are both popular medications that can help patients shed extra pounds in combination with a healthy diet and regular exercise. While Phentermine is a short-term drug that usually results in weight loss of approximately 14 lbs. over a maximum period of 24 weeks (but typically closer to 12), Mounjaro can lead to a total body weight reduction of 20% or more over a much longer period of time (clinical trials have lasted up to 72 weeks). In addition, it also helps to control blood sugar levels.

Which weight loss medication may be the right fit for your needs is something to discuss carefully with your doctor or healthcare provider. Both drugs can be effective under the right circumstances, but both also come with the possibility of inherent side effects and risks.

If you are looking to source high-quality weight loss medications at affordable prices, Bisonpharmacy.com can help. We specialize in shipping Canadian medications to your door at a fraction of the cost you’d pay at your local pharmacy. To order Mounjaro or Ozempic from Bisonpharmacy.com, contact us today!

About the Author

Ida Edlweiss Gumpal is a licensed Pharmacist and medical writer. She took her internships at Mercury Drug Corporation, Inc., a Hospital internship at De Vera Medical Center, Inc., and a Manufacturing internship at Philmed Laboratories, Inc. She has plans on attending medical school with the goal of specializing in Neurosurgery or Cardiothoracic surgery.