Mounjaro Yellow Card Reports: What They Are and What the Data Tells Us

You've had an unexpected reaction. Here's what happens next.

Picture the moment: a few days into a new dose, something doesn't feel right. Maybe it's stronger nausea than you expected, or a symptom you can't quite place. The first step is always to contact your prescriber or call 111 if you're worried, but there's a second step that many people overlook, and that's filing a Yellow Card.

The MHRA's Yellow Card scheme is the UK's national system for collecting reports of suspected adverse drug reactions. It works because real-world use of a medicine brings in a much wider, more varied population than any clinical trial: different ages, different combinations of other medicines, different underlying conditions. Trials for tirzepatide enrolled thousands of adults in controlled settings; the Yellow Card database captures what happens when millions of people use the medicine in their day-to-day lives.

Submitting a report takes around ten minutes on the Yellow Card website. You describe the reaction, when it started, the dose you were on, and any other medicines you take. You don't need a confirmed diagnosis. You don't need to be certain the medicine caused it. Suspicion is enough, the MHRA's pharmacovigilance team does the analytical work of separating signal from background noise. A single report rarely changes anything; a pattern across hundreds of reports absolutely can, and has done historically for many medicines.

There's a common misconception worth clearing up here: many people assume Yellow Card reports are only for serious or hospital-grade reactions. They're not. Mild nausea that made three weeks difficult, a skin reaction at the injection site, a persistent headache after titration, all of it is worth reporting, precisely because mild signals in large volumes can reveal something important that individual prescribers can't see from a small patient panel. You can also read more about what Mounjaro is and how it works if you'd like context before you report.

What the Black Triangle means for Mounjaro safety monitoring

Mounjaro received its UK licence for weight management relatively recently, and like all newer medicines it carries a Black Triangle (▼). This isn't a warning that the medicine is unsafe; it's a formal signal that additional monitoring is in place because long-term, large-scale post-market data is still accumulating. The designation is reviewed periodically and removed once regulators are satisfied the safety profile is well understood.

For patients this means one practical thing: the MHRA specifically asks healthcare professionals and patients to report all suspected adverse reactions, not just serious ones, for any Black Triangle medicine. That extra scrutiny is a feature, not a concern. It reflects the same rigour that produced the January 2026 Drug Safety Update on acute pancreatitis across GLP-1 medicines, a communication that reinforced existing guidance and reminded prescribers and patients to seek urgent medical attention for severe, persistent stomach pain that spreads to the back. That kind of safety communication emerges from pharmacovigilance data, including Yellow Card reports submitted by Mounjaro patients.

You can find background on the active ingredient in our tirzepatide overview. If you're currently on treatment through a regulated provider, your prescriber should be notified of any new or worsening symptoms as a matter of routine, that conversation and a Yellow Card report are complementary, not alternatives to each other.

How the MHRA uses Yellow Card data and what it tells us about Mounjaro so far

The MHRA publishes aggregated Yellow Card data through its interactive portal. For tirzepatide, reports began accumulating from 2023 onwards as private prescribing expanded ahead of any NHS rollout. The most commonly reported categories mirror what the clinical trials showed: gastrointestinal effects (nausea, vomiting, diarrhoea, constipation, reflux), injection-site reactions, fatigue, and headache. This alignment between trial data and real-world reports is reassuring, it suggests the medicine's profile is behaving as expected.

Where pharmacovigilance is most valuable is in detecting rarer signals. The MHRA's review process involves comparing the rate of a reaction in people taking Mounjaro against background rates in the general population, adjusting for the kind of people likely to be prescribed it. A reaction that sounds alarming in isolation may be unremarkable statistically; one that sounds trivial may cluster in a way that merits further investigation. That process depends entirely on reports coming in.

The NHS medicines page for tirzepatide lists the known side effects and guidance on when to seek help. For anything that feels urgent (severe stomach pain, signs of an allergic reaction, sudden vision changes) get medical help straight away rather than waiting to file a report. The report can follow; the Yellow Card scheme accepts retrospective submissions.

It's also worth knowing that counterfeit tirzepatide pens, which the MHRA has seized in large quantities from illegal sellers, carry a completely different risk profile. Any reaction from a pen obtained without a valid prescription and clinical assessment should be reported to the MHRA as a matter of urgency, and the source reported too. If you're researching how to access treatment safely, our guide to buying Mounjaro online legally covers what to look for in a regulated provider.

When and how to submit your own Yellow Card report

The process is simpler than most people expect. Go to yellowcard.mhra.gov.uk and choose the patient or healthcare professional route. You'll be asked for: the medicine name and dose, the reaction you experienced, when it started and how long it lasted, whether it resolved, and your other medications and relevant medical history. Anonymous reporting is possible, though including contact details allows the MHRA to follow up if needed.

Reports can also be submitted via the Yellow Card app, or by post if you prefer. Healthcare professionals (including the prescriber who reviewed your consultation) can submit on your behalf or alongside you.

At nume, our prescribers review every patient's progress before a repeat order is processed. If you raise a side effect with our clinical team, our support line is open seven days a week. We encourage reporting to the MHRA as well; the two things reinforce each other. If you're based in Wales and want to explore local access to treatment, you can find out more about getting Mounjaro in Cardiff through a regulated provider near you. If you haven't yet started treatment and want to understand whether it might be right for you, check your eligibility with a free consultation and a prescriber will walk through your full picture before anything is prescribed. Some patients also find it helpful to mark their progress milestones, and if you'd like to celebrate a treatment anniversary with someone, we have a Mounjaro birthday card you can send to a friend who is on their own weight loss journey.