Drug interactions — the short list that matters, and why it is shorter than you would expect

Patients arrive at this question expecting a long list, because most drugs have one. Peptides are different: they are broken down by ordinary proteolysis rather than processed through the cytochrome P450 system, which is where the great majority of drug interactions occur.

So the pharmacological list really is short. What replaces it is one mechanism that touches everything you swallow.

The one that can hurt you: insulin and sulfonylureas

This is the interaction with genuine acute consequences, and it applies only to people also being treated for diabetes.

GLP-1s lower blood glucose in a glucose-dependent way, which is why they cause very little hypoglycaemia on their own. Sulfonylureas — gliclazide, glipizide, glimepiride — and insulin do not work that way. They lower glucose regardless of what it is already doing.

Stack them and the risk of hypoglycaemia rises materially. Standard practice when adding a GLP-1 is to reduce the sulfonylurea dose, often substantially, or to stop it; insulin doses are usually cut as well, and then adjusted again at each escalation step.

The mechanism behind everything else: delayed emptying

The stomach empties more slowly, so oral medication reaches the small intestine — where absorption happens — later than it used to.

For most drugs this changes nothing that matters. A blood pressure tablet absorbed forty minutes later is still absorbed. For a small number of drugs, timing is the whole ballgame.

Narrow therapeutic index drugs are the concern: warfarin, digoxin, some anticonvulsants, lithium, and immunosuppressants such as tacrolimus. The advice is not to avoid them but to monitor more closely around starting and around each dose increase. If you take warfarin, expect your INR to be checked more often for a while.

Levothyroxine deserves a mention because so many people take it. Its absorption is famously sensitive to stomach conditions. The practical answer is consistency — same time daily, empty stomach, well away from calcium, iron and coffee — plus a thyroid function test a couple of months after starting or after a large dose step.

Oral drugs you need to work fast, such as analgesics, may simply take longer to act.

Oral contraceptives — and the tirzepatide asterisk

This is the interaction most likely to change someone’s life, and it applies to one molecule.

Mounjaro and Zepbound labelling advises that people using oral contraceptives switch to a non-oral method, or add a barrier method, for four weeks after starting the drug and for four weeks after each dose increase. With six dose steps, that is potentially six separate windows of reduced reliability.

Semaglutide carries no equivalent warning.

The full explanation is in the four-week rule, and the fertility side of it — restored ovulation in people who assumed they could not conceive — is covered in GLP-1s and PCOS and what happens to your cycle.

What does not interact

Worth stating positively, because unnecessary worry stops people taking medication they need.

• Antidepressants — SSRIs, SNRIs, mirtazapine, bupropion. No recognised interaction. Common combination. The mood picture is discussed in mood on a GLP-1.
• Statins. No interaction, and frequently co-prescribed given the cardiovascular indication.
• Blood pressure medication. No interaction, though doses often need reducing as weight falls — for good reasons.
• Proton pump inhibitors — omeprazole and relatives. Commonly used together for reflux.
• Metformin. No interaction. Standard combination.
• Antibiotics, in general.
• Most over-the-counter remedies, including the laxatives and anti-nausea preparations people reach for.

Alcohol, which is not an interaction but behaves like one

No metabolic interaction exists. The practical combination is still poor: alcohol slows gastric emptying further, irritates an already slowed stomach, worsens dehydration, and adds hypoglycaemia risk for anyone on insulin or a sulfonylurea.

Many people find they want considerably less of it anyway, which is one of the more interesting observations in this field and is covered in GLP-1s and alcohol.

The conversation to have

Bring the complete list — prescriptions, over-the-counter, and supplements — to whoever is prescribing. If you are not sure what is in your own cupboard, the brand-by-brand comparison is a reasonable place to audit it. Then ask three specific questions:

1. Does anything here need its dose reduced as I lose weight? Blood pressure medication, insulin, sulfonylureas and sometimes diuretics all do.
2. Is anything here narrow-window enough to need monitoring? Warfarin, levothyroxine, anticonvulsants, lithium.
3. If I am on tirzepatide, what am I doing about contraception for the next four weeks?

That covers essentially the entire practical field. It is also worth having the anaesthetic conversation recorded somewhere, since that one arrives without warning.

The reassuring conclusion

For most people the honest answer is that there is nothing to change and nothing to worry about.

The list of things that genuinely interact is short, well characterised, and concentrated in two groups: people on insulin or sulfonylureas, and people on tirzepatide using the pill. If you are in neither, this page is mostly reassurance.