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7 documented interaction groups for Mounjaro (tirzepatide). Severity rated. Management guidance per category.
GLP-1 delayed gastric emptying alters warfarin absorption variably. INR can shift up or down during titration weeks.
Slowed gastric emptying changes the rate (not extent) of warfarin absorption. Patients also typically experience reduced caloric intake → lower vitamin K consumption from leafy greens → INR drift.
Check INR weekly for the first 4 weeks AND for 2 weeks after each dose escalation. Adjust warfarin dose conservatively. Avoid changing other anticoagulant choices on top of GLP-1 initiation.
GLP-1s delay levothyroxine absorption AND increase its bioavailability — net effect is roughly 33% AUC increase. Patients may become slightly hyperthyroid if dose not reduced.
Slowed gastric emptying paradoxically increases levothyroxine AUC because absorption window is longer. Most patients see TSH drift downward (toward hyperthyroidism) by 2-3 months.
Recheck TSH 6-8 weeks after starting any GLP-1, then at 3 and 6 months. Take levothyroxine on completely empty stomach (1 hr before food, 4 hrs from other meds) — same rule as without GLP-1.
Note: Many patients fear the boxed thyroid C-cell tumor warning. That's about rodent medullary carcinoma — unrelated to Hashimoto's or autoimmune thyroid. Levothyroxine + GLP-1 is safe with TSH monitoring.
Tirzepatide (Mounjaro/Zepbound/Foundayo) reduces oral contraceptive absorption during initiation and dose escalation. Semaglutide does NOT have this effect.
Tirzepatide's gastric emptying delay specifically reduces ethinyl estradiol + levonorgestrel AUC by 20-30% during titration weeks. Semaglutide gastric effects do not affect OCP absorption at the same magnitude.
Use barrier method (condoms) for 4 weeks after starting tirzepatide AND for 4 weeks after each dose escalation. Consider switching to IUD or implant for long-term reliability. Switching to vaginal ring or patch is unaffected.
Note: CRITICAL DISAMBIGUATION: This applies only to TIRZEPATIDE products (Mounjaro, Zepbound, Foundayo). Semaglutide products (Ozempic, Wegovy, Rybelsus, Wegovy Pill) do NOT have clinically significant OCP interaction at usual doses.
Adding GLP-1 to sulfonylureas or insulin substantially increases hypoglycemia risk. Other diabetes meds need proactive dose reduction.
GLP-1s lower glucose. Stacking with insulinogenic drugs (sulfonylureas) or insulin itself causes hypoglycemia. Dose reduction is mandatory, not optional.
BEFORE starting GLP-1: reduce sulfonylurea dose by 50%. Reduce basal insulin by 20%. Reduce mealtime insulin proportional to expected appetite reduction. Monitor blood glucose 4x daily during titration.
Narrow-therapeutic-index immunosuppressants are at risk from GLP-1 gastric emptying delays. Trough levels can shift unpredictably.
Tacrolimus and cyclosporine have narrow therapeutic windows. Delayed absorption + altered first-pass metabolism shifts trough levels. Mycophenolate AUC can change.
Coordinate GLP-1 initiation with transplant team. Check immunosuppressant trough levels at weeks 1, 2, 4, then monthly. Consider conservative titration (longer 0.25mg phase). Some transplant centers contraindicate GLP-1 in first year post-transplant.
GLP-1s slightly delay statin absorption but clinical effect is minimal. As weight drops, LDL typically improves and statin dose may need reduction.
Delayed gastric emptying delays statin Cmax but total AUC is preserved. Therapeutic effect maintained. Major effect is weight-loss-driven LDL reduction → potential deprescribing opportunity.
Continue statin during GLP-1 titration. Check lipid panel at 3 and 6 months. If LDL drops >40 mg/dL with weight loss, discuss dose reduction or de-escalation with prescriber.
Some oral antibiotics interact with GLP-1s through additive gastric motility effects, potentially worsening nausea or interfering with absorption.
Macrolides (erythromycin, azithromycin) themselves accelerate gastric emptying — opposite of GLP-1 effect. Net result is unpredictable. Some other antibiotics absorbed less reliably when gastric pH/motility altered.
For short courses (5-10 days), continue both with careful symptom monitoring. For chronic prophylaxis, consult prescriber. Take antibiotics on same schedule each day. If severe nausea develops, separate GLP-1 dose by 1-2 days.
Editorial summary derived from FDA prescribing information and clinical pharmacology references. Not a substitute for prescriber review. Tell your prescriber and pharmacist every medication, supplement, and herb you take.