GLP-1s and Your Other Medications: What Actually Interacts

These drugs slow the stomach deliberately, so the obvious worry is that everything else you swallow will behave differently. Mostly it does not. The interactions that turn out to matter are stranger than the ones people ask about — one of them is mediated by what you eat, and the most practical one affects the GLP-1 itself rather than anything else in your cabinet.

The reassuring part, which is most of it

A dedicated set of clinical pharmacology studies gave healthy volunteers digoxin, warfarin, atorvastatin and a combination oral contraceptive, each with and without a GLP-1 injection, and measured what happened.^[1]

Total exposure was essentially unchanged for everything except atorvastatin, where it fell by 21%. Peak concentrations were generally lower and took longer to arrive — which is exactly what slowing the stomach would predict — but the conclusion was that no dose adjustment is recommended for any of the four.^[1]

The warfarin interaction is a dietary one

This is the finding worth carrying away. GLP-1 drugs do not affect warfarin’s pharmacokinetics — that has been tested directly and it holds.^[1] But warfarin is controlled by vitamin K intake, and these drugs reduce how much people eat. Less food means less vitamin K, and less predictable vitamin K means less predictable INR.

A 2026 study looked for exactly this in real-world records. From 53,943 patients screened, 1,021 had warfarin use documented both before and after starting a GLP-1. Mean time in therapeutic INR range fell from 64.2% to 62.1% — a drop of 2.1 percentage points, with a confidence interval excluding zero. Mean INR itself did not change at all.^[2]

The average INR held steady. What deteriorated was how much of the time it stayed where it was supposed to be.

That distinction is the whole result. An unchanged average with worse time-in-range means more swings in both directions, which is precisely the risk profile anticoagulation is trying to avoid. The effect is small — two percentage points — and it is a retrospective cohort rather than a trial. It is also entirely plausible mechanistically, and it argues for telling your anticoagulation clinic that you are starting a GLP-1, not for avoiding one.

If you take the oral version, this one is for you

Oral semaglutide is absorbed with the help of an added carrier molecule, and the arrangement is delicate. In a trial of 45 healthy subjects, taking five placebo tablets alongside it cut semaglutide exposure by 34%.^[3]

Read that the right way round. The concern is not what the GLP-1 does to your other pills — it is what your other pills do to the GLP-1. Someone taking a morning handful of medications with their tablet may be absorbing substantially less of the drug they are paying for, and would have no way of knowing except that it seems to be working less well than expected.

The same trial ran the reverse test with levothyroxine and found a 33% increase in total thyroxine exposure when the two were taken together, with the recommendation that thyroid parameters be monitored.^[3] Both drugs demand an empty stomach and a waiting period, which is why they collide in the first place.

What to actually do

1. Give someone the full list. A pharmacist can do this properly in a few minutes, and prescriptions written by different clinicians are exactly where interactions hide.
2. If you take warfarin, tell the anticoagulation clinic before you start. The mechanism is dietary, so it is your eating that changed, and they can monitor accordingly.
3. If you take levothyroxine, expect a thyroid recheck. Especially with the oral GLP-1, where the two compete for the same empty stomach.
4. If you take the oral GLP-1, ask about spacing. Other tablets taken with it can cut its absorption by a third.
5. Do not adjust anything yourself. Every number on this page belongs to the person who prescribed the drug it describes.