Phentermine and diethylpropion are two of the oldest appetite suppressants still talked about in the UK. Both work on the brain to dull hunger, both are controlled drugs, and neither holds a standard UK licence for weight loss. That last point surprises a lot of people.
If you have been searching for appetite suppressant tablets UK clinics offer, you have probably seen these two names side by side. They are similar, but not identical. Below we compare how they work, who might be considered for them, the side effects to watch for, and the newer licensed treatments that many UK patients now choose instead.
The short answer? Phentermine tends to be the stronger stimulant. Diethylpropion is often seen as the gentler of the two. Both are short term tools, not long term fixes, and both need proper medical supervision.
How phentermine and diethylpropion work
Both drugs belong to a family called sympathomimetic amines. In plain terms, they nudge the brain to release more noradrenaline, a chemical messenger that switches on your “fight or flight” system. When that system is active, hunger takes a back seat.
The result is a quieter appetite, smaller portions and fewer cravings between meals. Many people notice the effect within the first few days.
The drugs are close cousins, so the differences are about strength and feel rather than mechanism.
| Phentermine | Diethylpropion | |
|---|---|---|
| Also known as | Duromine, Adipex | Amfepramone, Tenuate |
| Stimulant strength | Stronger | Usually milder |
| Appetite effect | Very marked for most users | Noticeable, a little softer |
| Common complaints | Insomnia, jitters, racing heart | Dry mouth, restlessness, constipation |
| UK licence for weight loss | None | None |
| Typical use | Short courses only | Short courses only |
The main differences in practice
Phentermine is the one people describe as “switching hunger off”. That power comes at a price: it is more likely to cause sleep problems, a pounding heart and a wired, edgy feeling, especially in the first couple of weeks.
Diethylpropion tends to feel smoother. Some clinicians prefer it for patients who found phentermine too stimulating, or who are more sensitive to side effects. It still raises heart rate and blood pressure, though, so “milder” does not mean risk free.
On results, evidence for both is modest by modern standards. A 2009 study in the International Journal of Obesity found people taking diethylpropion lost around 10% of their body weight over 12 months, alongside diet changes. Phentermine trials show similar short term losses. The catch is that weight often returns once the tablets stop, unless habits have genuinely changed.
Are they legal in the UK?
Yes, but only in a narrow way. Neither drug has a current UK marketing authorisation for weight loss, so your GP will not prescribe them and they are not available on the NHS. Some specialist private clinics prescribe them as unlicensed medicines on a named patient basis, after a face to face or video assessment.
Both are also controlled drugs. Buying them from an unregulated website, a gym contact or a social media seller is illegal, and the tablets you receive could contain anything. If a site will post them to you after a five minute form, walk away.
Who might be considered
Clinics generally follow the same eligibility rules used for other prescription weight loss medicines:
- a BMI of 30 or above, or
- a BMI of 27 or above with a weight related condition such as high blood pressure or type 2 diabetes
Being eligible is only the first step. A responsible prescriber will check your blood pressure, heart rate, medical history and current medication before saying yes, then monitor you throughout.
Side effects and who should avoid them
The most common side effects are dry mouth, trouble sleeping, restlessness, headaches, constipation and a faster heartbeat. Less often, people experience mood changes, anxiety or a rise in blood pressure. Rare but serious risks include heart valve problems and pulmonary hypertension, plus a real potential for dependence.
You should not take either drug if you have:
- heart disease or uncontrolled high blood pressure
- an overactive thyroid
- glaucoma
- a history of drug or alcohol misuse
- certain antidepressants, especially MAOIs, in your system
- a pregnancy, or you are breastfeeding
Because of these risks, courses are kept short, usually a matter of weeks rather than months.
Licensed alternatives worth knowing about
Here is the honest truth: most UK patients who would once have been offered phentermine or diethylpropion now have better options. The newer GLP1 medicines are fully licensed by the MHRA, have far larger trials behind them, and tend to deliver bigger, longer lasting results.
Weekly injections
Mounjaro (tirzepatide) and Wegovy (semaglutide) are the two best known options. Each is a once weekly weight loss jab that mimics gut hormones to reduce appetite and slow digestion. In major trials, people on the top dose of Mounjaro lost around 20% of their body weight over 72 weeks, while Wegovy users lost around 15%. That level of Mounjaro weight loss is roughly double what older appetite suppressants achieve. You can compare Mounjaro and Wegovy injections side by side if you are weighing up which suits you.
Daily tablets
Not everyone likes needles. The MHRA approved the Wegovy pill in June 2026 and Lilly’s Foundayo (orforglipron) in August 2026, so Daily Weight Loss Tablets are now a real option. Foundayo has no food or water restrictions, and in trials the highest dose produced around 11% weight loss compared with 2% on placebo. For a closer look at the numbers, see our guide to Foundayo weight loss results.
If you would rather explore every oral option in one place, our full range of prescription weight loss tablets UK patients can access is a good starting point.
Which should you choose?
If you want the strongest evidence and the biggest average results, a Mounjaro injection is hard to beat. If you prefer a tablet, the newer GLP1 pills are generally a safer first choice than stimulant suppressants. Phentermine or diethylpropion may still suit a small group of people who want a short, closely supervised course, and who have been properly screened.
At Weight Medics, our clinicians look at your health, goals and budget before recommending anything, and we will tell you plainly if a treatment is not right for you.
FAQs
Which is stronger, phentermine or diethylpropion?
Phentermine is usually the stronger appetite suppressant and the more stimulating of the two. Diethylpropion tends to cause fewer jitters and sleep problems, which is why some people tolerate it better.
Can my GP prescribe phentermine or diethylpropion?
No. Neither drug is licensed for weight loss in the UK or available on the NHS. Only some specialist private clinics prescribe them, as unlicensed medicines, after a full assessment.
Is it safe to buy these tablets online without a consultation?
No. Both are controlled drugs, so buying them without a valid UK prescription is illegal. Unregulated sellers often supply fake or contaminated tablets, which can be dangerous.
How much weight can I lose?
Results vary, but most people lose a modest amount over a short course, often a few kilos a month at first. Weight tends to return after stopping unless you have built lasting habits alongside the medication.
Are the newer tablets better than phentermine?
For most people, yes. Licensed GLP1 tablets such as the Wegovy pill and Foundayo have stronger safety data and work for longer, without the stimulant side effects of older appetite suppressants.
Can I switch from phentermine to Mounjaro or Wegovy?
Often, yes. A clinician will review your health and timing, then advise on a safe gap or transition plan. Never combine weight loss medicines without medical advice.
