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What is tirzepatide? Zepbound, Mounjaro and how it works

Updated Sep 27, 2026. Numbers in brackets link to the source each fact comes from.

Tirzepatide is the active ingredient in Zepbound and Mounjaro, both made by Eli Lilly. Unlike semaglutide, it acts on two gut-hormone receptors, GIP and GLP-1, rather than one. Zepbound is the brand approved for weight loss and sleep apnea; Mounjaro is the brand approved for type 2 diabetes.

How it works

The Zepbound label calls tirzepatide an agonist of both the GIP receptor and the GLP-1 receptor [1]. GLP-1 helps regulate appetite and how much people eat, and early lab studies suggest the GIP part may add to that effect [2]. According to the label, tirzepatide cuts calorie intake, and the weight it removes is more fat than lean tissue [3].

Two brands, different approved uses

Brand Approved for
Zepbound Weight loss and keeping it off, in adults with obesity, or overweight plus a weight-related condition [4]
Zepbound Moderate to severe obstructive sleep apnea in adults with obesity [5]
Mounjaro Blood sugar control, with diet and exercise, in adults and children 10 and older with type 2 diabetes [6]
Mounjaro Lowering the risk of heart attack, stroke and heart-related death in adults with type 2 diabetes at high risk [7]

The FDA added the sleep apnea use to Zepbound’s label in a separate approval [26]. Mounjaro contains the same molecule as Zepbound, but Lilly’s own site says Mounjaro is not a weight loss drug [8]. Coverage rules can differ by brand too; see what each GLP-1 is approved for.

Forms

Zepbound comes as prefilled single-dose pens, single-dose vials, a multi-dose vial and a multi-dose KwikPen [18]. The KwikPen holds four weekly doses, one month’s supply in one device [19]. The form can change the price: LillyDirect, Lilly’s own pharmacy service, lists its self-pay prices for the vials and the KwikPen. Current prices are on the Zepbound page.

How it is taken

Zepbound is a shot under the skin once a week, at any time of day, with or without food [9]. You can inject it into the belly or thigh, or another person can inject it into the back of your upper arm; the label says to rotate sites each dose [10].

Everyone starts at 2.5 mg a week for 4 weeks [11]. That first dose is only for getting started, not for staying on [12]. After 4 weeks the dose goes to 5 mg, and it can rise in 2.5 mg steps, at least 4 weeks apart [13]. For weight loss the maintenance dose is 5 mg, 10 mg or 15 mg a week [14]; for sleep apnea it is 10 mg or 15 mg [15]. The top dose is 15 mg [16]. Mounjaro also starts at 2.5 mg and can rise in 2.5 mg steps when blood sugar needs more control, up to 15 mg for adults and 10 mg for children [17].

How much weight people lost in the label trial

The label’s first weight study ran 72 weeks in 2,539 adults with obesity, or overweight plus a related condition, and no diabetes [20]. Average body weight fell 15.0% at 5 mg, 19.5% at 10 mg and 20.9% at 15 mg, against 3.1% on placebo [21]. Trial results for other GLP-1s are on GLP-1 weight loss results.

Safety points on the label

  • A boxed warning: tirzepatide caused thyroid C-cell tumors in rats [22]. It is not for people with a personal or family history of medullary thyroid cancer or MEN 2 [23].
  • Do not combine it with another tirzepatide product or any GLP-1 medicine [24].
  • It needs a prescription; the FDA drug database lists the Zepbound KwikPen as a prescription product [25].

Side effects are compared drug by drug on GLP-1 side effects.

Compounded tirzepatide

Compounded drugs are not FDA approved and are not reviewed by the FDA for safety, effectiveness or quality before sale [28]. The FDA also says it knows of fraudulent compounded tirzepatide with false information on the label [27]. See fake and unapproved GLP-1 products.

Whether tirzepatide is right for you, and at which dose, is a decision to make with your prescriber.

Sources

  1. Tirzepatide is a GIP receptor and GLP-1 receptor agonist. dailymed.nlm.nih.gov, read Sep 27, 2026.
  2. GLP-1 is a physiological regulator of appetite and caloric intake. Nonclinical studies suggest the addition of GIP may further contribute to the regulation of food intake. dailymed.nlm.nih.gov, read Sep 27, 2026.
  3. Tirzepatide lowers body weight with greater fat mass loss than lean mass loss. Tirzepatide decreases calorie intake. dailymed.nlm.nih.gov, read Sep 27, 2026.
  4. to reduce excess body weight and maintain weight reduction long term in adults with obesity or adults with overweight in the presence of at least one weight-related comorbid condition. dailymed.nlm.nih.gov, read Sep 27, 2026.
  5. to treat moderate to severe obstructive sleep apnea (OSA) in adults with obesity. dailymed.nlm.nih.gov, read Sep 27, 2026.
  6. as an adjunct to diet and exercise to improve glycemic control in adults and pediatric patients 10 years of age and older with type 2 diabetes mellitus. dailymed.nlm.nih.gov, read Sep 27, 2026.
  7. to reduce the risk of major adverse cardiovascular (CV) events (CV death, non-fatal myocardial infarction (MI), or non-fatal stroke) in adults with type 2 diabetes mellitus who are at high risk for these events. dailymed.nlm.nih.gov, read Sep 27, 2026.
  8. Mounjaro is not a weight loss drug. mounjaro.lilly.com, read Sep 27, 2026.
  9. Administer ZEPBOUND once weekly at any time of day, with or without meals. dailymed.nlm.nih.gov, read Sep 27, 2026.
  10. Inject ZEPBOUND subcutaneously in the abdomen, thigh, or another person should inject in the back of the upper arm. Rotate injection sites with each dose. dailymed.nlm.nih.gov, read Sep 27, 2026.
  11. The recommended starting dosage of ZEPBOUND for all indications is 2.5 mg injected subcutaneously once weekly for 4 weeks. dailymed.nlm.nih.gov, read Sep 27, 2026.
  12. The 2.5 mg dosage is for treatment initiation and is not approved as a maintenance dosage. dailymed.nlm.nih.gov, read Sep 27, 2026.
  13. After 4 weeks, increase the dosage to 5 mg injected subcutaneously once weekly. The dosage may be increased in 2.5 mg increments, after at least 4 weeks on the current dose dailymed.nlm.nih.gov, read Sep 27, 2026.
  14. Weight Reduction and Long-Term Maintenance: 5 mg, 10 mg, or 15 mg injected subcutaneously once weekly. (2.2) dailymed.nlm.nih.gov, read Sep 27, 2026.
  15. Obstructive Sleep Apnea: 10 mg or 15 mg injected subcutaneously once weekly. (2.2) dailymed.nlm.nih.gov, read Sep 27, 2026.
  16. The maximum dosage of ZEPBOUND for all indications is 15 mg injected subcutaneously once weekly. dailymed.nlm.nih.gov, read Sep 27, 2026.
  17. The recommended starting dosage of MOUNJARO is 2.5 mg injected subcutaneously once weekly [see Dosage and Administration (2.2)]. If additional glycemic control is needed, increase the dosage in 2.5 mg increments after at least 4 weeks on the current dose. The maximum dosage of MOUNJARO is: 15 mg injected subcutaneously once weekly in adults. 10 mg injected subcutaneously once weekly in pediatric patients. dailymed.nlm.nih.gov, read Sep 27, 2026.
  18. ZEPBOUND (tirzepatide) is a clear, colorless to slightly yellow solution available in cartons containing 4 pre-filled single-dose pens, 1 single-dose vial, 4 single-dose vials, 1 multi-dose vial, or 1 single-patient-use KwikPen as follows: dailymed.nlm.nih.gov, read Sep 27, 2026.
  19. KwikPen is a multi-dose pen that holds four doses of Zepbound medicine, one for each week of the month. lilly.com, read Sep 27, 2026.
  20. Study 1 (NCT04184622) was a 72-week trial that enrolled 2,539 adult patients with obesity (BMI ≥30 kg/m2), or with overweight (BMI 27 to <30 kg/m2) and at least one weight-related comorbid condition dailymed.nlm.nih.gov, read Sep 27, 2026.
  21. % Change from baselineb -3.1 -15.0 -19.5 -20.9 -3.2 -12.8 -14.7 dailymed.nlm.nih.gov, read Sep 27, 2026.
  22. In rats, tirzepatide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures. dailymed.nlm.nih.gov, read Sep 27, 2026.
  23. A personal or family history of MTC or in patients with MEN 2 dailymed.nlm.nih.gov, read Sep 27, 2026.
  24. ZEPBOUND contains tirzepatide. Coadministration with other tirzepatide-containing products or with any glucagon-like peptide-1 (GLP-1) receptor agonist is not recommended. dailymed.nlm.nih.gov, read Sep 27, 2026.
  25. ZEPBOUND KWIKPEN TIRZEPATIDE 60MG/2.4ML (25MG/ML) SOLUTION;SUBCUTANEOUS Prescription accessdata.fda.gov, read Sep 27, 2026.
  26. Today, the U.S. Food and Drug Administration approved Zepbound (tirzepatide) for the treatment of moderate to severe obstructive sleep apnea (OSA) in adults with obesity fda.gov, read Sep 27, 2026.
  27. FDA is aware of fraudulent compounded semaglutide and tirzepatide marketed in the U.S. that contain false information on the product label. fda.gov, read Sep 27, 2026.
  28. However, compounded drugs are not FDA approved. This means the agency does not review compounded drugs for safety, effectiveness or quality before they are marketed. fda.gov, read Sep 27, 2026.

This page reports what official sources say; it is not medical advice. Medical disclaimer.