Someone walked into my personal training studio in Hampstead last week asking about Clenbuterol. He’d read about it on a forum. He wanted to know what dose to run.
I asked him 2 questions. First, did he know Clenbuterol is a veterinary drug licensed for racehorses with respiratory problems? He didn’t. Second, did he know that the “fat loss” people report on Clen is roughly 20% actual fat, and the rest is water, glycogen, and muscle? He had no idea.
That conversation isn’t unusual. Every 6 months or so Clenbuterol has another moment in the fitness world, usually when a reality TV personality posts a “shredded in 8 weeks” transformation. The internet lights up. The forums fill with dosing protocols. And nobody, not a single person, explains what the drug actually does inside the body.
Expertise reviewed by Giuseppe Marchesani, Senior Coach at Body Transformation London. Level 3 Personal Trainer.
So let’s do that properly.
What Clenbuterol is
Clenbuterol is a beta 2 adrenergic agonist. In veterinary medicine it’s used as a bronchodilator for horses with respiratory distress, because it opens the airways. In humans, it’s not licensed in the UK for anything. It’s not a scheduled drug, but it’s also not a medicine you can legally be prescribed here. If you have it, you’ve bought it from a dodgy website or an underground supplier, which means you have no idea what dose you’ve actually got.
That matters, because Clen has an exceptionally long half life of around 36 hours. A wrong tablet doesn’t wear off by evening. It sits in your system for a day and a half.
What It Actually Does
The thermogenic effect, the one everyone chases, is real but modest. Clen raises your core body temperature slightly, which does increase caloric expenditure. In practical terms we’re talking about 150 to 300 extra calories a day, if you’re lucky. You could achieve the same effect with a 20 minute walk after dinner and avoid most of what follows.
What Clen is far more effective at is making your heart work harder. Beta 2 agonism increases heart rate, raises blood pressure, and over time causes cardiac hypertrophy, where the heart muscle thickens in a way that’s not functional. Autopsy studies on long term Clen users have shown the kind of cardiac remodelling you’d expect in someone decades older.
The second thing it does, and this is the one nobody talks about, is cause meaningful muscle loss. Beta 2 agonism burns through muscle glycogen fast, and the drug has a known catabolic effect at the doses people actually run. The classic Clen “transformation” is 10kg lost in 6 weeks. In my clinical experience, pulled apart by scans, that’s typically 2kg of fat, 2 to 3kg of water and glycogen, and 5kg of muscle. You end up lighter on the scale and visibly weaker.
Then there’s everything else. Tremors severe enough that clients can’t hold a coffee cup. Insomnia for the full 6 weeks of the cycle. Electrolyte depletion that’s put people in A&E. Anxiety and panic attacks. Chronic cramping.
The Honest Comparison
If you want to lose body fat and keep your muscle, the clinical answer is boring: a moderate caloric deficit of 400 to 500 calories below maintenance, 1.8 to 2.2g of protein per kilogram of bodyweight, and resistance training 3 to 5 times a week. Sleep. Steps. Patience.
That protocol loses 0.5 to 1kg of fat per week, keeps every gram of muscle you’ve built, protects your heart, and leaves you looking better in 6 months than any Clen cycle leaves you looking in 6 weeks. It’s the same protocol we use for weight loss coaching at BTX, built around InBody 770 scans and personalised blood work with Dr Matt Milner.
If you’ve got 10kg of fat to lose, the system above will take it off without a horse drug. If you’ve already run Clen and you’re feeling the heart palpitations, the tremor or the insomnia, stop now, and get your bloods and your ECG checked.
Want a personal trainer in London to take you through it? Book a free strategy call at bodytransformationlondon.com, or DM “STRONG” on Instagram @jamescooperbtx. Not in London? Our online personal training runs the same system, wherever you are.