Mounjaro in the BNF: how tirzepatide is classified and what prescribers check

Step 1, What the BNF actually is and why it shapes every Mounjaro prescription

The British National Formulary is the UK's authoritative clinical reference for medicines, published jointly by the British Medical Association and the Royal Pharmaceutical Society and updated regularly. Every registered prescriber in the UK is expected to use it. When a GP, independent prescriber or specialist considers initiating tirzepatide, the BNF entry is the structured checklist they work through: indication, dose, cautions, contra-indications, interactions and monitoring.

The BNF tirzepatide entry sits within the antidiabetics section and cross-references NICE guidance, including NICE TA1026, which recommends tirzepatide for weight management in eligible adults. That cross-reference matters: it signals that evidence, not just marketing, underpins the listing. For patients, the practical effect is that any legitimate prescriber (whether in an NHS clinic or a regulated online service) is consulting the same document before they approve, amend or decline a prescription.

It is also worth knowing the BNF is a professional reference, written for clinicians rather than patients. The NHS patient-level guidance on tirzepatide covers broadly the same ground in plain language and is the better first read if you want to understand what the medicine does and how to use it safely. If you would like a closer look at what the BNF says about tirzepatide specifically, including how the entry is structured and what each section means for prescribers, we have a dedicated page that walks through it in detail.

Step 2, The cautions and contra-indications prescribers review before approving

Before a prescriber can issue a Mounjaro prescription in line with the BNF, they must satisfy themselves that none of the listed cautions or contra-indications apply. This is not a formality: it is the clinical assessment that separates a safe prescription from an unsafe one, and it is the reason these medicines are prescription-only.

The BNF flags several cautions a prescriber will consider. A personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 represents a clear contra-indication. A history of pancreatitis requires careful thought; the MHRA reinforced this in January 2026 with a Drug Safety Update reminding prescribers and patients that acute pancreatitis is a known, if uncommon, serious risk. Severe stomach pain that spreads to the back is the symptom to take seriously, and anyone experiencing it should seek urgent medical attention rather than waiting for a scheduled review.

The BNF also notes cautions around diabetic retinopathy, severe gastrointestinal disease and dehydration risk from prolonged vomiting or diarrhoea. Women using oral contraceptives should add a non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide's effect on gastric emptying can reduce pill absorption, a detail the prescriber is expected to raise at initiation. Pregnancy, breastfeeding and trying to conceive sit outside the licensed indications; the medicine is not recommended in those circumstances.

At nume, every consultation is read by a GPhC-registered independent prescriber who works through exactly these criteria before a prescription is issued or declined.

Step 3, Doses, titration and what the BNF says about monitoring

The licensed dose range runs from 2.5 mg to 15 mg once weekly. The BNF sets out a starting dose of 2.5 mg, which exists not because it is therapeutically effective at that level but because it gives the body time to adjust and reduces early nausea. From there, the prescriber titrates in roughly four-weekly steps as the patient tolerates: 5 mg, then 7.5 mg, and so on up to the 15 mg maintenance dose if that is appropriate. The 5 mg pen is a common first increase, and prescribers working within a service like ours review tolerability and weight response before each step up.

On monitoring, the BNF points prescribers toward body weight, glycaemic markers where relevant, and any emerging signs of the cautions noted above. NICE TA1026 adds a practical checkpoint: if a patient has not achieved at least 5% weight loss after six months at the highest tolerated dose, continuing treatment should be reviewed. That is not a BNF instruction but it sits within the same evidence framework prescribers use, and a good clinical service will flag it proactively. You can read more about the full tirzepatide evidence base if that context is useful.

For patients wondering about specific dose availability or the differences between pen formats, those details are confirmed at the consultation stage rather than listed in advance, stock and clinical suitability need to align. If you are already established on treatment elsewhere, the transfer process follows a similar clinical review.

Step 4, How this translates to accessing Mounjaro through a regulated route

The BNF is the framework; the consultation is where it meets real life. The NHS route to tirzepatide is subject to the phased eligibility criteria set out under NICE TA1026, which means many people who could benefit from treatment do not yet qualify or face a wait. A private prescription through a regulated online pharmacy is a legal, parallel route, provided the clinical assessment is genuine, the prescriber is registered, and the medicine comes through the licensed UK supply chain. Patients based in Northern Ireland should also be aware that accessing Mounjaro in Northern Ireland involves some specific considerations around supply and eligibility that differ from the rest of the UK.

Checking a pharmacy on the GPhC register takes about a minute and tells you whether it is authorised to dispense. Sellers offering these medicines without a proper clinical assessment, or at prices so low they cannot cover a prescription, are a safety risk, fake pens have been seized in large quantities across the UK, and the MHRA has repeatedly warned that counterfeit products can be dangerous. The guide to buying Mounjaro online safely covers what to look for.

If you want to understand how treatment might fit your situation, the weight-loss treatment overview sets out the options. When you are ready to take the next step, a free consultation with our prescribers is the place to start, there is no charge, no commitment, and the clinical review happens the same day for orders placed before 12pm. Some people find it easiest to get this sorted at the start of the month, before the week fills up.