What else you are taking, and why these drugs change it

The interaction profile of this class follows from the mechanism rather than from metabolism. These drugs are not enzyme inhibitors — the Wegovy label notes that semaglutide “has very low potential to inhibit or induce CYP enzymes, and to inhibit drug transporters.” What they do instead is slow the stomach, and everything below follows from that. The mechanism itself is in How GLP-1 drugs actually work, and what that explains.

The general rule both labels state

Wegovy: it “delays gastric emptying” and “may impact absorption of concomitantly administered oral medications,” with increased clinical or laboratory monitoring advised for oral medicines that have a narrow therapeutic index or that require monitoring. Zepbound says the same and names an example: “Monitor patients on oral medications dependent on threshold concentrations for efficacy and those with a narrow therapeutic index (e.g., warfarin).”

One detail from Lilly’s label is worth holding on to, because it explains the timing of everything else here: “Tirzepatide delays gastric emptying. The delay is largest after the first dose and this effect diminishes over time.” The interaction risk is therefore front-loaded — largest at initiation and after each step up the ladder in Every dose step, and what to do when you miss one, not constant.

The interactions each label quantifies

The levothyroxine figure deserves emphasis for anyone considering the pill over the injection. A daily tablet taken on an empty stomach, followed by a thirty-minute wait before any other oral medicine, is awkward enough for someone on morning thyroid replacement without a 33% change in exposure on top. That trade-off is worked through in Pills or injections, now that the same drug comes as both.

Hypoglycaemia, if you take something else for blood sugar

Both labels carry the same warning: concomitant use with insulin or an insulin secretagogue such as a sulfonylurea may increase the risk of hypoglycaemia, including severe hypoglycaemia, and reducing the dose of the insulin or secretagogue may be necessary. Both also instruct that blood glucose be monitored in patients with diabetes before and during treatment.

This is the interaction most likely to matter to someone who arrived through a weight-management service rather than a diabetes one, because the service may not be managing the other prescription — or know about it. Whoever adjusts the insulin has to be someone who knows both.

What this means for choosing a service

An intake that does not ask for your full medication list cannot apply any of the above. That is not a stylistic complaint — it is a documented interaction going unchecked. 84 of the 87 services reviewed here describe how their consultation works and 78 describe who they will and will not treat, but what a questionnaire actually asks is visible only once you are inside it. If the form never mentions your other prescriptions, that is the answer. It is item two and item five on Thirteen things a GLP-1 service should tell you before you pay.

And if what arrives is a compounded vial rather than a labelled pen, none of the interaction data above was generated for the product in your fridge. Compounded preparations are not FDA-approved and carry no approved labelling at all — Brand-name vs compounded GLP-1s and How to tell a regulated GLP-1 seller from a risky one.