Mounjaro Vs Phentermine: Which is Best for Weight Loss

Mounjaro Vs Phentermine

Mounjaro Vs Phentermine is a comparison article of these two drugs although both of these drugs have not been investigated in a head-to-head comparison trial.

Mounjaro is a once-weekly injectable diabetes medicine. It has just been approved by the FDA for obesity as Zepbound injections.

Phentermine, on the other hand, is a drug that suppresses appetite and boosts metabolism.

Phentermine is available in the markets under the brand names Adipex-P and Suprenza. It was approved by the FDA for the short-term treatment of weight loss in addition to diet and exercise.

However, the FDA now recommends using Phentermine for weight loss only in combination with a topiramate combo drug that is available in the markets under the brand name Qsymia.


The Basic Differences Between Mounjaro and Phentermine:

Mounjaro is a newly approved first-in-its-class dual incretin mimetic drug. It is also called Twincretin as it enhances the action of both GLP-1 and GIP in the body.

Mounjaro is used as a once-a-week subcutaneous injection. It is initiated in a dose of 2.5 mg once weekly for four weeks and then titrated every four weeks by 2.5 mg up to a maximum dose of 15 mg per week.

It is approved only for use in Type 2 Diabetic individuals and is considered the most potent diabetes medicine.

After its approval as a weight loss drug on 8th November 2023, it is now the most potent weight loss drug as it lowers body weight by up to 25%.

Phentermine is an orally disintegrating tablet that gets dissolved readily when kept over the tongue. It is available in three different strengths 15, 30, and 37.5 mg as Phentermine Hydrochloride (equivalent to 12, 24, and 30 mg of phentermine).

It is a stimulant and appetite suppressant drug, approved for weight loss in combination with Topiramate as Qsymia.

Phentermine Vs Mounjaro: Basics




ClassSympathomimeticDual GLP-1 and GIP analog
Drug/BrandAdipex-P, SuprenzaTirzepatide
ManufacturerTeva, AkrimaxEli Lilly
Uses (Approved Indication)Weight Loss (with Topiramate)Diabetes Type 2

Weight Loss

One major difference between Mounjaro and Phentermine (Adipex-P) is that Mounjaro is a prescription-only medicine while Phentermine (Adipex-P) is available over the counter (OTC medicine).

Phentermine is sold @ 39 USD on Amazon while one monthly cost of Mounjaro is around USD 1100 without insurance and USD 35 with Insurance.

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How effective is Mounjaro Vs Phentermine for Weight Loss?

Tirzepatide (Mounjaro) and phentermine have not been studied in a head-to-head comparison. The weight loss effects presented here are indirect comparisons vs placebo.

In a study performed in a Mexican obese otherwise healthy population, Phentermine in a dose of 15 mg and 30 mg was evaluated for 3 to 6 months.

The weight loss effects ranged from 4 to 8 kgs as presented here in the table below:

Weight loss [Ref]

Phentermine 15 mg

Phentermine 30 mg

Weight lost at 3 months-4.5 kg-5.8 kg
Weight lost at 6 months-6.9 kg-8.4 kg

The weight loss effect of Mounjaro was also evaluated in obese individuals.

The study included more than 2500 participants who were obese or overweight with at least one obesity-related medical condition like hypertension, dyslipidemia, sleep apnea, etc excluding diabetes [Ref])

Mounjaro significantly lowered body weight ranging from 15 to 21% at different doses. The picture below summarizes the weight loss effects of Mounjaro vs Placebo.

percentage weight loss with mounjaro
Percentage weight loss with Mounjaro

Mounjaro Vs Phentermine: Weight loss difference vs Placebo:

Mounjaro is considered more potent than Phentermine. In fact, Phentermine is not approved as a weight loss drug. It is only approved for weight loss with Topiramate as Qsymia.

However, the table below is given for the sake of comparison to determine the difference in weight loss between Mounjaro and Phentermine:

Mounjaro Vs Phentermine: Weight Loss Difference Vs Placebo




Weight Loss
  • 5 mg: -5.3 kg
  • 10 mg: -6 kg
  • 15 mg: -6.8 kg
-5.3 kg [Ref]
Percent weight loss
  • 5 mg: -11.9 %
  • 10 mg: -16.4 %
  • 15 mg: -17.8 %

(at 72 weeks [Ref])


(at 28 weeks [Ref])

The weight loss effects of Mounjaro 5 mg and Phentermine are probably comparable. However, as the dose of Mounjaro is increased, it is associated with significantly greater weight loss than Phentermine.

From the table above, it may be concluded that Mounjaro is associated with 7% to 13% greater weight loss than Phentermine.

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Mounjaro Vs Phentermine Side Effects and Drug Interactions:

Mounjaro (Tirzepatide) is way more safe than Phentermine. Apart from the gastrointestinal side effects associated with Mounjaro, it is not associated with significant side effects.

On the other hand, Phentermine is a stimulant drug. It has got an addictive potential. Like other stimulant drugs, it may be misused by people to achieve euphoria and become overactive. In addition, it is also associated with withdrawal symptoms.

However, the big difference in terms of side effects is that Phentermine may cause angina, hypertension, and worsen heart failure. On the other hand, Mounjaro may protect your heart.

Here is a table summarizing the side effects of Mounjaro vs. phentermine:

Mounjaro Vs Phentermine Side Effects and Contraindications

Side effects


  • Cardiac diseases (angina, heart failure, stroke, hypertension, arrhythmias)
  • Hyperthyroidism
  • Glaucoma
  • Agitation
  • Drug dependence in the past
  • Allergic reactions
  • Pregnancy and Breastfeeding
  • Use with MAO Inhibitors
Side effects of Mounjaro and Phentermine
  • Hypoglycemia
  • Pancreatitis
  • Renal failure
  • Nausea
  • Vomiting
  • Diarrhea
  • Abdominal pain
  • Constipation
  • Gallstones
  • Amylase elevation
  • Lipase elevation
  • Injection-site reactions
  • Fatigue
  • Palpitations
  • Pulmonary hypertension
  • Drug Tolerance
  • Abuse and dependence
  • Withdrawal effects
  • Palpitations
  • Hypertension
  • Overstimulated, agitation, and euphoria
  • Psychosis
  • Headaches
  • Dry mouth
  • constipation or Diarrhea
  • Altered taste
  • Urticaria
  • Loss of libido
  • Impotence

There are certain drugs that should be avoided when you are using Mounjaro or Phentermine. The table below summarizes these drug interactions:

Mounjaro Vs Phentermine Drug Interactions

Drug Interactions


  • Bexarotene
  • Gatifloxacin
  • MAO Inhibitors
Monitor when given in combination
  • Insulin
  • Oral Contraceptives
  • Oral antidiabetic medications
  • Warfarin
  • Corticosteroids
  • Diuretics
  • Fenofibrate
  • Salmeterol
  • Antidepressants (MAO inhibitors)
  • Alcohol
  • Insulin
  • Oral hypoglycemic drugs
  • Sympathomimetics
  • Adrenergic blockers

It is important to note that since Mounjaro is a drug used for diabetes, most drug interactions are related to anti-diabetics.

Furthermore, Mounjaro delays gastric emptying and may impair the absorption of some important medicines like levothyroxine and warfarin.

Phentermine is a stimulant drug and if given within 14 days of using an MAO inhibitor, it may result in a hypertensive crisis. In addition, the patient may need to adjust his/ her blood pressure medications when starting Phentermine for weight loss.

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In Conclusion:

Mounjaro has been approved by the FDA for Diabetes Type 2 and obesity. It is way more potent than Phentermine for weight loss.

In addition, Mounjaro is safer but costlier than Phentermine. In fact, Phentermine is also available as an OTC medicine.

What do you think?

Written by Dr. Ahmed

I am Dr. Ahmed (MBBS; FCPS Medicine), an Internist and a practicing physician. I am in the medical field for over fifteen years working in one of the busiest hospitals and writing medical posts for over 5 years.

I love my family, my profession, my blog, nature, hiking, and simple life. Read more about me, my family, and my qualifications

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