Who a Microdose Start Is For (BMI 27+, New to GLP-1) and Who Decides

Last updated: September 17, 2026

Written by: Jay Hastings, CEO of PlexusDx, and Jayden Lee, PharmD, EMBA, PlexusDx Medical Director

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Director, with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

This article is part of the PlexusDx Education Hub — your resource for evidence-based guidance on Peptides & GLP-1. Browse all Peptides & GLP-1 education

Three questions arrive together: what is the starting dose of tirzepatide, what is the lowest dose, and who qualifies for a low start? This article answers each with the label and with the PlexusDx study’s inclusion rules. It does not tell you whether you qualify. A licensed prescriber does that. Microdosing is not a medical term. In the PlexusDx study it meant a total weekly dose of 1 or 2 mg of injectable tirzepatide, below the usual 2.5 mg starting dose.

The labeled starting dose

Per the FDA-approved labels, Zepbound® and Mounjaro® start at 2.5 mg once a week for four weeks, then move to 5 mg. The label describes 2.5 mg as a starting dose to help the body adjust, not as a treatment dose. The label’s maximum is 15 mg a week. Zepbound® is approved for chronic weight management in adults with a BMI of 30 or higher, or 27 or higher with at least one weight-related condition, and for obstructive sleep apnea in adults with obesity. Zepbound® and Mounjaro® are registered trademarks of Eli Lilly and Company. PlexusDx is not affiliated with or endorsed by Eli Lilly and Company.

The lowest dose

The lowest dose of the FDA-approved products is 2.5 mg. There is no 1 mg or 2 mg Zepbound® or Mounjaro® pen. Doses below 2.5 mg exist only as compounded medication, prepared by state-licensed compounding pharmacies operating in FDA-registered facilities, when a licensed prescriber orders them. Compounded medication is not an FDA-approved product.

Who was in the PlexusDx study

The study set three main rules for who counted. Adults. New to GLP-1 medicines, meaning no prior GLP-1 use, no transfer from another program, and no restart. And a starting BMI of 27 kg/m² or higher. Sixty patients met those rules and the study’s timing rules.

Who they were: mean age 47.3 years (SD 11.0). Forty-nine of 60 (81.7%) were women. Mean starting BMI 33.3 kg/m² (SD 6.1); median 31.4 (IQR 28.7–36.1). Mean starting weight 205.7 lb (SD 47.6). Fifty-three of 60 started at 1 mg a week in total and 7 of 60 at 2 mg a week in total.

These are the rules the study used to pick records to look at. They are not eligibility rules for care. Meeting them does not mean a prescriber will start you low, and not meeting them does not mean you cannot be treated.

Who decides

A licensed prescriber, every time. In the PlexusDx program, an independent network of U.S.-licensed clinicians through a state-licensed telehealth platform reviews your intake, your health history, and your goals, then decides whether to prescribe, at what weekly total, and how to follow you. Things that can matter: your BMI, other health conditions, other medicines, past reactions to medicines, and your own preferences about side effects and pace.

You can ask about a low start. You can also ask why a clinician prefers the labeled start. Either way the answer should be specific to you.

“Can I stay on the lowest dose?”

People ask this a lot. The PlexusDx study cannot answer it. No patient in the study had an eligible weight at 90 days or later, and the study did not look at long-term safety. What it can say is that some patients stayed at 1 mg a week during the windows studied, some moved to 2 mg a week, and once anyone went above 2 mg their later weights left the analysis. Whether a low dose is right for months or years is a decision for you and your prescriber, revisited over time.

Related: what is a microdose of tirzepatide and how the microdose tirzepatide protocol works.

About the PlexusDx study

A retrospective, single-organization observational study of adults documented as new to GLP-1 medicines with BMI ≥27 kg/m²; no comparison group; self-reported weights; not a clinical trial; preprint not yet peer reviewed. The records came from the commercial care program that PlexusDx operates. Care was delivered by an independent network of U.S.-licensed clinicians through a state-licensed telehealth platform. The medication was compounded injectable tirzepatide with vitamin B12 at a total weekly dose of 1 to 2 mg, prepared by state-licensed compounding pharmacies operating in FDA-registered facilities. The study was written by Jay Hastings (PlexusDx CEO) and Jayden Lee, PharmD, EMBA (PlexusDx Medical Director). See the PlexusDx microdose tirzepatide study (60 patients, 2026).

The authors’ conclusion, in their words: “These hypothesis-generating observations support prospective comparative study, not conclusions about causality, equivalent dosing, or long-term safety.” The full preprint is on medRxiv: Weight Loss With Microdose Tirzepatide: Real-World Outcomes in Patients Initiating Treatment Below 2.5 mg Weekly (doi:10.64898/2026.09.15.26363172; posted September 17, 2026; CC BY 4.0). It has not yet been peer reviewed.

Where PlexusDx fits

PlexusDx offers compounded tirzepatide through its Weight Management Protocols, including the entry-tier Microdose GLP-1 Protocol. You fill out a short intake. A licensed clinician reviews it and decides what, if anything, fits you. The study described here looked back at records. It is not a promise of any result.

This article is educational. PlexusDx offers compounded tirzepatide through its Weight Management Protocols. The GLP-1 & Peptide Pathways Genetic Test analyzes how your genes influence peptide-related biological pathways. It does not recommend, prescribe, or determine which peptides you should use. Consult a qualified healthcare provider before beginning any peptide protocol. Compounded tirzepatide is not an FDA-approved drug product. It is prepared by state-licensed compounding pharmacies under federal compounding rules and is not the same as Zepbound® or Mounjaro®. Zepbound® and Mounjaro® are registered trademarks of Eli Lilly and Company. PlexusDx is not affiliated with or endorsed by Eli Lilly and Company.

Frequently Asked Questions

What is the starting dose of tirzepatide?

Per the FDA-approved labels, Zepbound® and Mounjaro® start at 2.5 mg once a week for four weeks before moving to 5 mg. In the PlexusDx study of compounded tirzepatide, patients started lower, at a total of 1 or 2 mg a week, as ordered by a licensed prescriber.

What is the starting dose of compounded tirzepatide?

Whatever a licensed prescriber orders; there is no label for compounded doses. In the PlexusDx study, 53 of 60 patients started at 1 mg a week in total and 7 of 60 at 2 mg a week in total. Those describe one program’s records, not a rule for you.

What BMI do you need for a microdose start?

The PlexusDx study only included adults with a starting BMI of 27 or higher, because that matched the study’s question. It is a research rule, not a care rule. Whether you are a candidate for any dose is decided by a licensed prescriber who reviews your full history.

What is the lowest dose of tirzepatide?

The lowest FDA-approved dose is 2.5 mg a week; no 1 mg or 2 mg Zepbound® or Mounjaro® pen exists. Lower amounts exist only as compounded medication ordered by a prescriber and prepared by a state-licensed compounding pharmacy. Compounded medication is not an FDA-approved product.

Can I stay on the lowest dose of tirzepatide?

That is a prescriber’s decision, reviewed over time. The PlexusDx study cannot answer it because no patient had an eligible weight at 90 days or later and long-term safety was not studied. Some patients stayed at 1 mg a week within the study windows; others moved to 2 mg.

This article is part of the PlexusDx Education Hub. Browse all Peptides & GLP-1 education

Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance.

Commercial transparency: PlexusDx offers genetic testing, blood biomarker testing, personalized supplement recommendations, and related precision wellness services. Product mentions are intended to help readers understand available options and should not be interpreted as medical advice.

Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.