Who Can Take Weight-Loss Medicines? The Rules Clinicians Use to Decide
Updated: Oct 10
Weight-loss medicines like Wegovy and Mounjaro are in the news a lot. Many people want to try them. But not everyone can get them, and not everyone should. Clinicians follow careful rules to decide who is a good fit. Let's look at how those rules work, using the UK's NHS as our main example.
What Is BMI, and Why Does It Matter?
BMI stands for "body mass index." It's a number clinicians calculate using your height and weight. It gives a rough idea of whether someone's weight might be affecting their health. It isn't perfect — it can't tell the difference between muscle and fat, for example — but it's still the main number used to decide who can get weight-loss medicine.
Private Prescriptions: A Different, Faster Route
Not everyone gets these medicines through the NHS. In the UK, some people choose a private prescription instead, through a private clinic or an online pharmacy.
Private prescriptions use a wider BMI rule than the NHS. Usually, a BMI of 30 or higher qualifies on its own, with no extra health condition needed. If your BMI is 27 or higher, you can often still qualify, as long as you have at least one weight-related condition, such as high blood pressure or type 2 diabetes. That's a noticeably lower bar than the NHS uses, which is one reason private options have become so popular.
The process usually moves fast, since there's no waiting list like the NHS has. It typically works like this: you fill in an online health questionnaire, a registered prescriber reviews your answers, and — because regulators require pharmacies to properly check your height and weight rather than simply trust a number you type in — you'll usually need a video call or some other proof, not just a photo. If everything checks out, the medicine can arrive within one to two weeks.
The trade-off is cost. Private prescriptions aren't paid for by the NHS, so people cover the price themselves. As of 2026, that's roughly £80 to £150 a month at the lowest starting dose, climbing to around £270 to £350 a month at the highest doses. For many people, that monthly cost is just as big a decision as the medical one.
The NHS Route: Why the Bar Is Higher
In England, the NHS uses BMI as a first checkpoint too, but the bar is set higher than the private route. For semaglutide (the medicine in Wegovy), you usually need a BMI of 30 or higher. For tirzepatide (the medicine in Mounjaro), the bar is a bit higher: a BMI of 35 or higher.
There's an important detail here. People from South Asian, Chinese, Middle Eastern, Black African, or African-Caribbean backgrounds often start having weight-related health problems at a lower BMI than the general guideline suggests. So the NHS lowers the BMI cutoff by 2.5 points for these groups. This isn't about treating anyone differently for no reason — it's based on research showing health risks can begin earlier for these groups at the same BMI number.
It's Not Just About the Number
A high BMI alone usually isn't enough. Most NHS rules also require at least one "weight-related comorbidity." A comorbidity is another health condition linked to someone's weight — things like high blood pressure, high cholesterol, sleep apnea (a condition where breathing stops and starts during sleep), heart disease, or type 2 diabetes.
Clinicians look for this second condition because it shows the extra weight isn't just a number — it's already affecting the body. Medicine is offered when the benefits are likely to outweigh the risks and cost, and having a related health problem helps show that.
Why Can't Everyone Get It Right Away?
Here's something surprising: even people who qualify might have to wait. In the UK, a huge number of people meet the criteria — roughly 3.4 million, according to NHS England. But there aren't enough specialist clinicians and clinics to treat everyone at once safely.
So the NHS is rolling this out in stages, almost like a queue sorted by need. Right now, priority goes to people with the most health conditions and the highest BMI. Over the next few years, the criteria loosen step by step, letting more people in. NHS leaders have said that opening access to everyone overnight could overwhelm hospitals and clinics that also need to offer support like nutrition advice and check-ups — not just the injection itself.
When a Medicine Isn't Right, No Matter the BMI
Separately from access rules, there are medical reasons a clinician might say no to these medicines for a specific person. Clinicians call these "contraindications" — reasons a treatment could be unsafe for someone, based on their personal or family health history.
The clearest example: anyone with a personal or family history of a rare thyroid cancer called medullary thyroid cancer, or a genetic condition called MEN2, should not take these medicines. This warning exists because, in animal studies, these drugs caused thyroid tumors. Scientists haven't proven this happens in humans, but clinicians avoid the risk entirely.
Other reasons a clinician might say no include a past episode of pancreatitis (a painful swelling of the pancreas, an organ that helps digest food), being pregnant or trying to become pregnant, or having had a serious allergic reaction to the medicine before.
There are also situations where clinicians are extra cautious rather than saying an outright no — for example, someone with a history of an eating disorder, severe stomach-emptying problems called gastroparesis, or gallstones. In these cases, the clinician and patient talk it through together and decide what's safest.
The Bottom Line
Being eligible for weight-loss medicine involves two separate checks. One is about access: does your BMI and health history meet the criteria your health system uses, and is a treatment slot available? The other is about safety: does your personal medical history rule out these drugs for you specifically? Both matter. A good clinician will walk through each one honestly, rather than just handing out a prescription because someone asked for it. That carefulness is exactly what makes these medicines something to trust rather than fear.
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