What is Clenbuterol?
Clenbuterol is a beta-2 agonist, originally an asthma medicine to widen the airways. In the cutting corner it is used for its fat-burning and appetite-suppressing effect. It is explicitly not a steroid and not an anabolic in the classic sense.
Up front: Clenbuterol noticeably loads the heart and circulation. The fat-burning effect is real. So is the price for it.
Comparison
Fat loss compounds in the lexicon
What each entry targets, as stated in the TPD lexicon. Tap a card to open its entry.
Simplified. Salbutamol uses the same receptor as clenbuterol but acts shorter. Yohimbine blocks a brake receptor, T3 acts via thyroid hormone receptors, DNP directly in the mitochondria. 11
How it works
Clenbuterol activates beta-2 receptors and through them ramps up metabolism: more heat production (thermogenesis), more fat burning, dampened appetite. At the same time heart rate and partly blood pressure rise, the flip side of the same signal.
The body adapts relatively fast (the receptors become less sensitive), which is why the effect fades after one to two weeks.
Where it acts
Where clenbuterol acted in studies
Tap a point or a card. Colour = beta-2 receptor, grey = observed.
- Airways
Widens the bronchi, hence the approval as an asthma drug8
β2 - Skeletal muscle
More muscle protein and lean mass, the signal fades with continued use1
β2 - Fat metabolism
Fat oxidation +39% after one dose, fat mass unchanged after 2 weeks2
Observed in trials - Heart
Case reports: racing heart, atrial fibrillation, heart muscle damage7
Observed in trials
Targets in the ChEMBL database11
Compound ChEMBL CHEMBL49080: one recorded mechanism, Agonist.
How to take it
Oral, dosed in micrograms (mcg), not in mg.
- Usually in the morning, because the stimulating effect can disturb sleep.
- With some water, with or without food.
- Because of the adaptation, a pattern like two weeks on, two weeks off is often run.
Dosing
Dosing
Dosing, titration and syringe
Loading the dosing information …
- Getting started and stepping up, drawn as a staircase
- Units on the U-100 syringe for every step
- How long one vial lasts
What the research shows
A recent human study (Hostrup, 2025) showed that Clenbuterol does build lean mass and muscle protein, but at the same time worsens endurance fitness and dulls the beta-2 signals in muscle. That sums up the clenbuterol dilemma well: a bit of muscle and fat effect, paid for with worse conditioning and heart strain.
In animal models the muscle-building and fat-lowering effect is clear, in humans it is weaker, while the side effects remain.
Study results
What the trials measured
Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.
Endurance after 2 weeks1
11 healthy men, crossover against placebo
Values from the abstract. Plus 0.91 kg more lean mass, but no change in fat mass, sprint power, heart muscle mass or blood volume. Fat burning in muscle was slowed.
About two hours after one dose2
6 young men, change from the value before the dose
Values from the abstract, without a placebo group. Lactate rose by 90 percent, carbohydrate oxidation stayed the same.
Glucose uptake into muscle under insulin34
Compared with placebo, healthy men 2 weeks, overweight people 4 weeks
Values from the abstracts. The value for the front thigh was not statistically certain (p = 0.072). The study in healthy men reported no serious side effects.
Muscle disease Pompe disease5
Phase 1/2, on top of enzyme therapy, 11 of 13 completed the study
Values from the abstract. The placebo group showed no significant changes. Mild, transient side effects were raised muscle enzyme values, cramps and tremor.
Research status
Where the research stands
ClinicalTrials.gov lists 12 trials with clenbuterol. 9 are shown here. Two with unknown status are missing, a Pompe disease trial was withdrawn without participants. As of 26 Oct 2026. 10
- Preclinical Animal studies: more muscle mass in many species
- Phase 1 2 trials, 0 completed, 1 stopped early
- Phase 2 6 trials, 5 completed
- Phase 3 no registered trial
- Approval as an asthma drug in some countries, not in the US
Show all 7 trials on a timelineHide timeline
NCT01942590 · 17 participants, started Sept 2013, primary endpoint reached Sept 2016.
Registry entry ↗ · Results: Koeberl et al., Molecular Therapy, 2018
NCT03800290 · 11 participants, started Jun 2019, primary endpoint reached Apr 2021.
Registry entry ↗ · Results: van Beek et al., Nature Communications, 2023
NCT04245709 · 25 participants, started Feb 2020, primary endpoint reached Mar 2021.
Registry entry ↗ · Results: Li et al., Journal of Clinical Neuromuscular Disease, 2023
NCT04739423 · 41 participants, started Jun 2021, primary endpoint reached Jul 2022. Tests several agents.
Registry entry ↗ · Results not yet in PubMed
NCT04921306 · 14 participants, started Aug 2022, primary endpoint reached Mar 2024.
Registry entry ↗ · Results: Van Lier et al., Nature Communications, 2026
BetaSBMA · NCT06169046 · 90 participants (planned), started Apr 2024, primary endpoint planned Jun 2027.
NCT03860870 · 16 participants, started Mar 2019, primary endpoint reached Dec 2022.
Registry entry ↗ · Results not yet in PubMed
Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026. Phase 1: 2 trials on safety and pharmacokinetics, not shown as bars.
Side effects
- Shaking (especially of the hands), racing heart, palpitations.
- Restlessness, sleep problems, sweating.
- Muscle cramps, often linked to loss of magnesium and potassium.
- Longer term, strain on the heart muscle, the central risk.
Tolerability
Tolerability in the ALS study
This many of 25 participants had side effects or stopped because of them. Most common were tremor, cramps, insomnia and stiffness.
The dose was increased up to the highest study dose. Case reports in athletes describe racing heart, atrial fibrillation, heart muscle damage, heart attack and deaths.67
Practice
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Ease in slowly and watch heart rate.
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Ensure enough magnesium, potassium and fluids (cramps).
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Do not take late in the day or sleep suffers.
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With heart problems or high blood pressure, leave it alone.
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Banned and detectable in official competition.
Storage
Store dry, cool and protected from light, out of reach of children. Keep the original container closed and mind the expiry date.
