The GLP-1 problems that reach your clinic are rarely the headline ones. They are a sulfonylurea that now causes hypoglycemia, a levothyroxine dose that is suddenly too high, an INR that drifts, vitamin D that falls and lean mass that goes with the fat. Microdosing, the newest trend, has no trial behind it.
Key points
- Hypoglycemia < 54 mg/dL: 10.3% with tirzepatide plus a sulfonylurea vs 2.1% without [1].
- Oral semaglutide increased levothyroxine exposure by 33% [2]. With warfarin, time in range fell 2.1 points [5].
- In a review of 35 trials, a median 28.3% of weight lost was in muscle-related indices (fat-free mass, lean mass or muscle), and no study measured physical function objectively [6].
- Microdosing: “high-quality evidence is essentially absent” [9].
US status (October 2026): Semaglutide (Ozempic, Wegovy, Rybelsus) and tirzepatide (Mounjaro, Zepbound) are FDA-approved, and both shortages have ended. See the semaglutide and tirzepatide status pages.
Four interactions get missed
- Sulfonylureas and insulin: on the tirzepatide label, hypoglycemia < 54 mg/dL was 10.3% with a sulfonylurea vs 2.1% without [1].
- Levothyroxine: with oral semaglutide, “levothyroxine exposure was increased 33%” [2]. A case on subcutaneous semaglutide after thyroidectomy needed a 25% levothyroxine dose reduction for suppressed TSH [3].
- Warfarin: in 1,021 patients, time in therapeutic range fell 2.1 points after starting a GLP-1 [5]. The tirzepatide label asks for monitoring of narrow-therapeutic-index drugs [1].
- Oral contraceptives: one 5 mg dose of tirzepatide reduced ethinyl estradiol Cmax by 59% [1].
Slower gastric emptying does not always reduce exposure to oral drugs. A 2026 review found increased exposure to thyroxine, atorvastatin and ramipril on some labels, and warns that “a reduction in exposure should not be assumed” [4].
- 10.3%: hypoglycemia with a sulfonylurea vs 2.1% [1]
- 33% higher: levothyroxine exposure with oral semaglutide [2]
- 28.3%: median share of weight lost in muscle-related indices (35 trials) [6]
Muscle and bone go with the fat
A systematic review of 35 trials (Ann Intern Med 2026) found that a median 28.3% of weight lost on incretins was in muscle-based indices such as fat-free mass, lean mass or muscle (IQR 15.9 to 39.9%). 65% of studies exceeded the benchmark of about 25%. The bigger gap: no study reported objective physical function [6].
For bone, in a four-arm trial, liraglutide alone was associated with lower hip and spine bone mineral density than exercise alone. With liraglutide plus exercise, bone density did not change versus placebo [7].
Micronutrient deficiencies are common
A 2026 review pooled six studies with 480,825 adults. Vitamin D deficiency was the most common abnormality: 7.5% at 6 months and 13.6% at 12. Ferritin was 26 to 30% lower than with SGLT2 inhibitors, and more than 60% consumed less calcium and iron than required [8]. The data are mostly observational and do not prove causation [8].
Microdosing has no trial behind it
Patient communities, telemedicine programs and clinician anecdotes popularized using 25 to 50% of the recommended starting dose or longer intervals, even “counting clicks” on pens. A 2026 commentary sums it up: “high-quality evidence is essentially absent” [9]. Another paper links the trend to “research-grade” peptides bought online and to med spas [10]. MDside’s fuller take is in GLP-1 microdosing, and the sourcing problem is covered in research-use-only peptides.
What this means for you
Build these checks into the intake and the titration visit.
- Before starting or increasing the dose, review sulfonylureas and insulin [1], levothyroxine [2][3], warfarin [5] and oral contraceptives [1].
- During treatment, address protein, resistance exercise and micronutrients, which is where studies show losses [6][7][8].
If you are building or auditing a program, our weight management service runs on this checklist. For the compounding rules after the shortages, see compounded GLP-1s after the shortage.
References
- Eli Lilly and Company. Zepbound (tirzepatide) US prescribing information, revised 08/2026. Link
- Novo Nordisk. Wegovy (semaglutide) US prescribing information, revised 06/2026. Link
- Wilcox L, Van Dril E. Suppressed thyroid stimulating hormone levels after initiation of a subcutaneous glucagon-like peptide-1 receptor agonist in a post-thyroidectomy patient managed with levothyroxine case report. J Am Pharm Assoc (2003). 2024;64(6):102185. PMID 38992739. Link
- Chung JY. Motility-mediated drug interactions with GLP-1 receptor agonists: labeled evidence and a framework for quantitative assessment. Transl Clin Pharmacol. 2026;34(3):129-138. PMID 42846711. Link
- Gilbert SJ, et al. Evaluating the indirect interaction between glucagon-like peptide-1 receptor agonists and warfarin using real-world data. J Thromb Thrombolysis. 2026;59(6):1519-1527. PMID 42115581. Link
- Batsis JA, et al. Effect of incretin-based and nonpharmacologic weight loss on body composition: a systematic review. Ann Intern Med. 2026;179(7):996-1013. PMID 41996180. Link
- Jensen SBK, et al. Bone Health After Exercise Alone, GLP-1 Receptor Agonist Treatment, or Combination Treatment: A Secondary Analysis of a Randomized Clinical Trial. JAMA Netw Open. 2024;7(6):e2416775. PMID 38916894. Link
- Urbina J, et al. Micronutrient and Nutritional Deficiencies Associated With GLP-1 Receptor Agonist Therapy: A Narrative Review. Clin Obes. 2026;16(1):e70070. PMID 41549912. Link
- Beck H, Clements JN. Considerations and challenges in microdosing of GLP-1-based receptor agonists. Expert Opin Pharmacother. 2026:1-4. PMID 42788826. Link
- Trainer N. The “microdosing” dilemma: Balancing patient anecdotes with clinical safety amid GLP-1 compounding restrictions. J Am Assoc Nurse Pract. 2026. PMID 42201545. Link
This is general information, not medical or legal advice. Rules vary by state and change. Confirm your own facts with counsel.