Clenbuterol dosage and evidence
Written by Aaron CuhaReviewed Sep 2026
The short answer
The first proper human study, in 2025, found two weeks of clenbuterol added 0.91 kg of lean mass, did nothing to fat, and cut fitness (the most oxygen the body can use) by 7 percent. It is not approved for people in the United States. It has poisoned people in outbreaks, from tainted meat and from heroin cut with it.
Evidence grade for Clenbuterol: Limited. The rule: people have taken it in a published study, but fewer than 100 of them. The sum on this page: 2 completed trials, 11 people given it. A grade counts how much has been done to people, not whether it works.
- People given it
- 11
- Human studies
- 2
- Found nothing
- 2
Dose
No human dose established
Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose.
2 completed trials, 11 people given it
5 effects reported, 4 unwanted, 1 wanted, by people using it, not a trial
Goal pages: Fat loss and metabolism, Muscle and strength, Heart and circulation
In plain English
In plain English, from Aaron
This is how I would explain it to a friend. The evidence, with the numbers, is further down the page.
Clenbuterol is an airway drug for horses in the US, and for people in a few other countries. It is sold illegally as a fat burner. It speeds up the heart and makes you shake. It is banned in sport.
What people take it for
- Burning fat fast.
- Keeping or adding lean muscle while dieting.
What studies in people showed
The only proper study had 11 healthy men. They took it for two weeks and a dummy pill for two weeks. On the drug they gained 0.91 kg of lean mass. Fat did not change at all. Their fitness fell 7 percent. In a food poisoning outbreak in Spain, 113 people got sick from tainted liver. They had racing hearts, shaking and muscle aches. Heroin cut with it sent 34 people to US emergency rooms. Six showed heart muscle damage.
What people report
Reports, not study results
The good
- People describe feeling it work at first, before it fades within a few weeks of steady use.
The bad
- Cramps are the most common complaint. People drink lots of water and take taurine to cope.
- Hand shaking, bad sleep, heavy sweating, headache and raised blood pressure come up again and again.
- People argue about whether anything can stretch out the useful window before it fades.
- Users describe serious accidents, including a seizure at a high dose in a light person.
Where these come from: The good and bad lines are forum patterns from Bluelight threads shown on this page. They are not from a study. The lean mass and fitness numbers are from the study of 11 men on this page.
My bottom line
The one real study found it did not touch fat and made fitness worse. It puts a big load on your heart. I would not use it. Lose fat with food, training and sleep, and check your blood pressure while you do.
An opinion, not a finding. I am a coach, not a doctor.
What it has been measured to do
Measured in people
Goals Clenbuterol has been tested for in people, biggest studies first. We count only what the studies on this page say, so the real numbers may be higher.
- Losing fat specifically11 people · 1 study · 1 found nothing
- Keeping or building muscle18 people · 2 studies · 2 found nothing
- Lasting longer, stamina11 people · 1 study · 1 found nothing
- Losing weight7 people · 1 study · 1 found nothing
- Blood pressure7 people · 1 study · 1 found nothing
- Heart attacks and strokes7 people · 1 study · 1 found nothing
- Blood sugar and insulinno headcount stated · 1 study
What people using it report
Real people telling you what happened to them. Nobody checked it, and the people who post are not a fair sample, so it cannot prove anything works. It is still the best guide on this page to what people actually do with Clenbuterol, what they expect, and what goes wrong. The full account, including the negative reports, is below.
27 of the 36 indexed goals have no study of any kind behind Clenbuterol
No study in people, animals or cells on this page reports a result for these. That does not prove Clenbuterol does nothing for them. It does mean anything you have heard about Clenbuterol and one of these did not come from a study cited here.
Show full evidence
What is Clenbuterol?
The first proper human study, in 2025, found two weeks of clenbuterol added 0.91 kg of lean mass, did nothing to fat, and cut fitness (the most oxygen the body can use) by 7 percent. It is not approved for people in the United States. It has poisoned people in outbreaks, from tainted meat and from heroin cut with it.
Clenbuterol is a long-acting drug that opens the airways by switching on beta-2 receptors. A few countries license it for people with breathing problems. Others use it only for animals. In the United States it is neither: the FDA has not approved it for people, and the US product is a horse medicine.
For decades the muscle claim rested on rats. In 2025 a team in Copenhagen ran the study nobody had run. Eleven healthy men took 80 micrograms a day by mouth for two weeks, and a dummy pill for two weeks, with a three-week break in between. On the drug, lean mass rose by 0.91 kg and muscle protein went up. Fat mass did not move at all. That is worth stopping on, because fat loss is why most people buy it. The most oxygen the body could use fell by 7 percent, and exercise capacity by 4 percent. Muscle enzymes that burn fat were turned down. And the signal that drove the muscle gain faded over the two weeks.
The harm record is not theory. In 1992, 113 people in Catalonia, Spain, were poisoned by veal liver from cattle illegally fed clenbuterol. They had a racing heart, shaking, aching muscles and headache, lasting up to six days. Between 2005 and 2015, heroin cut with clenbuterol sent clusters of people to US emergency rooms. In one outbreak, 34 people across five states had racing hearts, low blood pressure, low potassium and high lactate, and six showed blood signs of heart muscle damage. A second, separate cluster of 13 people hit Richmond, Virginia.
In animals, clenbuterol makes the heart muscle thicken. In the one human study that checked, two weeks of dosing did not change how much muscle the heart's main pumping chamber had. Two weeks is too short to settle that, and nobody has run a longer study.
How does Clenbuterol work?
It switches on beta-2 receptors. In muscle, that sets off a chain of signals inside the cell. In the 2025 human study that chain was strongly switched on, along with a growth signal that drove the lean mass gain. The same receptor type in the heart and blood vessels causes the racing heart, shaking and low potassium seen in poisonings. Repeated doses make the receptor go numb. In that study the signal faded across the two weeks. That is the science behind the on-and-off cycling users worked out by trial and error.
What have studies in people found about Clenbuterol?
There is real human data: one controlled study where healthy men took the drug and a dummy in turn, one small study in heart failure patients, a study of how the body handles the drug, and three separate public health investigations of mass poisoning.
- Spain, 1992: 113 people poisoned by tainted veal liver. Symptoms lasted up to 6 days. Nobody died.
- United States, 2005 to 2007: 34 emergency room visits from heroin cut with clenbuterol. 6 of the patients showed heart muscle damage.
- Richmond, Virginia, 2015: 13 more patients poisoned the same way.
- 11 healthy men took 80 micrograms a day for 2 weeks, and a dummy for 2 weeks, in turn: 0.91 kg more lean mass, no change in fat, the most oxygen the body could use down 7 percent, exercise capacity down 4 percent, sprint power unchanged.
- 7 heart failure patients on a heart pump took up to 720 micrograms a day for 3 months: lean mass and thigh strength rose. Heart pumping and exercise capacity did not improve.
- How the body handles it: half is gone from the blood after about 35 hours, so a daily dose builds up for several days before it levels off.
What this does not tell you: The only controlled study ran for two weeks in eleven young men. It cannot tell you what months of use do, and months is what buyers actually do. Heart muscle mass did not change in those two weeks, so the heart thickening seen in animals is neither confirmed nor ruled out in people. No study has tested clenbuterol for fat loss in people who want to lose fat, and the one study that measured fat found no effect. The poisoning reports describe sudden overdoses in people who did not choose the dose. They show what poisoning looks like, not what regular self-dosing does.
What do people who took Clenbuterol report?
This is what real people say happened to them, good and bad. Nobody measured it. A report cannot tell a real effect from the lift people get just from expecting it to work, from a diet or training change made at the same time, or from what was really in the vial. We still put it near the top. It is the next best thing to a study in people: what happens when real people use it, including what goes wrong.
- Described amounts start around 20 to 40 mcg and are stepped up every few days, with experienced users describing 80 to 160 mcg and occasional higher figures. Two weeks on and two weeks off is the most repeated pattern, and morning dosing is preferred to limit sleep disruption.
- Cramps are the most common complaint, described as severe without heavy fluid intake, and several grams of taurine a day is the countermeasure people describe to each other.
- Hand tremor bad enough to interfere with writing, insomnia, heavy sweating, headache, breathlessness during training, raised blood pressure and a sense of dull thinking all appear repeatedly.
- Tolerance is a central theme. People describe whatever they were after falling away within a few weeks of continuous use, and disagree about whether add on drugs meant to restore the body's response make it work for longer.
- In a harm reduction setting it is described as having no recreational appeal whatsoever, only discomfort, which makes the volume of complaint reports in these threads comparatively hard to dismiss.
- Serious accidents are described, including a seizure attributed to overheating at a high amount in a light person, alongside repeated warnings against measuring from a supplied dropper and against handling raw powder.
- It is generally described as not muscle sparing on its own, which is why it is almost always discussed alongside other compounds rather than by itself.
Sources: Clenbuterol threads on Bluelight, found by site-restricted search and read directly, September 2026. Nothing above is attributed to any specific post and no post is reproduced.
The state of the research
Clenbuterol is a rare case where what users claim and what the study found point in opposite directions on the main selling point. It is sold mostly as a fat burner. The only controlled study that measured body makeup found no effect on fat and a clear gain in lean mass. It also found a drop in aerobic fitness that no endurance athlete would accept. The authors concluded that the side effects alongside the muscle gain justify its ban in sport.
The two ways people get poisoned are worth keeping apart. Meat gets tainted where clenbuterol is used illegally to bulk up cattle and pigs. That has caused outbreaks in Spain, China and elsewhere. It is also the standard excuse of athletes who test positive. Heroin cut with clenbuterol is a different problem, seen in at least two US clusters. There the drug gets into the street supply, and users get doses far above any medical range.
The heart question is still genuinely open. At high doses, beta-2 drugs thicken the heart in animals, and the heart damage markers in the heroin outbreak are real. The one human study built to look at heart structure ran for two weeks and found nothing. No sign at two weeks is not proof of safety at two years.
The evidence, in charts
Fig. 1a · how many people
144people, across 4 studies in people cited here
| Study | People | Share |
|---|---|---|
| Public Health Reports 1995 | 113 | 78% |
| Journal of Public Health Management and Practice 2017 | 13 | 9% |
| The Journal of Physiology 2025 | 11 | 8% |
| The Journal of Heart and Lung Transplantation 2006 | 7 | 5% |
One study holds 78% of these people, so the total rests more on one study than its size suggests. Counts are the number of people each study on this page says it signed up. That includes studies that only watched people and gave nobody the compound. 2 more studies in people give no headcount and are not counted here. Studies still signing people up are left out, and only studies cited on this page are counted.
Fig. 1b · evidence mix
6 of 7 sources here are studies in people. The rest is not.
- Study in people · done in people686%
- Animal study · done in animals114%
Counted from the list of sources on this page. It counts only what we cite, so there may be more studies out there. A review counts once, however many studies it sums up.
Fig. 2 · evidence over time
The studies cover 7 different years, 1985 to 2025, counted from the list of sources on this page.
Fig. 3 · legal status at a glance
US
Not approved
UK
Not approved
AU
Not approved
CA
Not approved
Not approved in any of the four countries shown. This is what the law says, not a verdict on the science. See Legal status below for the full text and any notes.
Fig. 4 · how much does what
Nobody has tested different amounts in people
This chart would show how the effect changes as the amount goes up. For this compound, nobody has measured that in people. So we leave it empty. We will not borrow an animal chart and pretend it works the same in people.
Fig. 5 · what the molecule is
- Kind of compound
- Small molecule
- Molecule weight
- 277.2 Da as the free base; the hydrochloride salt used in products is 313.7 Da
- Half-life
- About 35 hoursLong, 1 to 7 days
- Chain length
- None on file
Half-life as stated on file: About 35 hours in plasma in healthy volunteers, which is long for a beta-2 agonist and is why effects and detection both persist for days.
No amino acid sequence is on file for Clenbuterol, which is expected: a small molecule is not built from amino acids.
The studies we cite
- Study in people2025Clenbuterol induces lean mass and muscle protein accretion, but attenuates cardiorespiratory fitness and desensitizes muscle β2-adrenergic signalling
Clenbuterol does build muscle in people, and it wrecks your fitness while it does. 11 healthy young men took the drug for two weeks and a dummy for two weeks, in turn, with three weeks between. Lean mass rose by just under a kilogram. Fat mass did not move at all, which matters because fat loss is why most people buy it. The most oxygen their bodies could use fell 7 percent, and exercise capacity fell 4 percent. The muscle response faded across the two weeks.
The first randomised controlled trial. 11 healthy men aged 18 to 40, two 2-week cycles of oral clenbuterol at 80 micrograms a day versus placebo with a 3-week washout. Lean mass +0.91 kg (95% CI 0.02 to 1.81, p < 0.05), no effect on fat mass, maximal oxygen uptake -7% (p < 0.001), exercise capacity -4% (p < 0.001), no change in left ventricular mass, blood volume or haemoglobin mass. Muscle protein content rose while 3-hydroxyacyl CoA dehydrogenase activity and OXPHOS complex V fell. The beta-2 signalling response declined across the 2 weeks.
- Study in people1995Epidemiologic study of an outbreak of clenbuterol poisoning in Catalonia, Spain
Clenbuterol in food poisoned 113 people in Spain in 1992. The source was veal liver from cattle illegally fed the drug. More than half of those affected had jitteriness, a racing heart, muscle tremors, aching muscles and headache. Symptoms started anywhere from fifteen minutes to six hours after eating and lasted from ninety minutes to six days. The drug turned up in their urine. Nobody died. This is what a small accidental dose does to ordinary people.
113 cases of poisoning in 1992 traced to veal liver from cattle given clenbuterol. More than half had nervousness, tachycardia, muscle tremors, myalgia and headache. Onset 15 minutes to 6 hours, symptoms lasting 90 minutes to 6 days. Urine clenbuterol 11 to 486 parts per billion in 47 samples. No deaths.
- Study in people2008A descriptive study of an outbreak of clenbuterol-containing heroin
Clenbuterol mixed into street heroin sent 34 people to emergency rooms across five US states. They arrived with racing hearts, low blood pressure, high blood sugar, low potassium and raised lactate. Six of them had blood evidence of damage to the heart muscle. The drug was confirmed in their blood. Nobody chose these doses and nobody was measuring anything in advance. This is a hazard report, not a study of the drug.
34 probable or confirmed emergency department presentations across 5 US states over 6 months. Blood clenbuterol 2.4 to 26 ng/mL. Significant tachycardia, hypotension, hyperglycaemia, hypokalaemia and raised lactate; 6 of the patients had biochemical evidence of myocardial injury.
- Study in people2017Collaborative Public Health Investigation of Clenbuterol-Adulterated Heroin Outbreak-Richmond, Virginia, March-April 2015
A second heroin contamination outbreak, this one confined to a single city. 13 patients met the case criteria and clenbuterol was confirmed in samples taken from them. Every case traced back to one supplier in one part of Richmond, Virginia. The cluster stopped when that supplier was arrested. This is public health detective work rather than a study of what the drug does.
A second, later outbreak. 13 patients met clinical and epidemiological criteria, clenbuterol confirmed in clinical specimens, all linked to one supplier in one part of Richmond. The cluster stopped when the supplier was arrested.
- Study in people2006Effect of clenbuterol on cardiac and skeletal muscle function during left ventricular assist device support
Clenbuterol built leg muscle in heart failure patients and did nothing for their hearts. 7 people supported by a mechanical heart pump took rising oral doses for three months, up to a very high daily amount. Body weight and lean mass rose, and thigh strength improved from about 37 to about 46 kilograms of force. The pumping fraction did not improve and the heart chamber got larger. No serious side effects or rhythm problems appeared, though a muscle enzyme rose in four patients.
7 heart failure patients on a left ventricular assist device given oral clenbuterol up-titrated to 720 micrograms a day for 3 months. No serious adverse events or arrhythmias, creatine phosphokinase rose in 4 patients. Ejection fraction did not improve and end-diastolic dimension increased; body weight and lean mass rose and quadriceps maximal voluntary contraction improved from 37.0 to 45.8 kg. Cardiac function did not improve.
- Study in people1985Pharmacokinetics of plasma and urine clenbuterol in man, rat, and rabbit
This study measured where clenbuterol goes in the body, not what it does. Healthy volunteers took single oral doses at three strengths. Blood levels peaked within about two and a half hours, and the body takes roughly 35 hours to clear half of it, which is long. Most of it sticks to blood proteins. About a fifth came out unchanged in urine within three days. Taken twice a day, levels flattened out after four days. Nothing about benefit or harm was tested here.
Oral doses of 20, 40 and 80 micrograms in healthy volunteers. Peak plasma levels within 2.5 hours, plasma half-life about 35 hours, plasma protein binding 89 to 98 percent, and about 20 percent of the dose recovered unchanged in urine by 72 hours. Twice-daily dosing reached plateau within 4 days.
- Animal study1991Effects of clenbuterol on skeletal muscle mass, body composition, and recovery from surgical stress in senescent rats
In old rats. One of the old animal studies the muscle claim rested on for three decades, before any human study existed.
Rats. One of the old animal studies the muscle claim rested on for three decades before a human randomised trial existed.
Common questions
Has Clenbuterol been tested in people?
Yes. 11 people given it across 2 completed trials cited here, 2 of which found nothing. The first proper human study, in 2025, found two weeks of clenbuterol added 0.91 kg of lean mass, did nothing to fat, and cut fitness (the most oxygen the body can use) by 7 percent. It is not approved for people in the United States. It has poisoned people in outbreaks, from tainted meat and from heroin cut with it.
What is the Clenbuterol dosage?
No human dose established. Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose. 2 completed trials, 11 people given it
Is Clenbuterol FDA-approved?
Not approved for human use in the United States, at any dose or for any use. Its only US approval is for animals, as a drug that opens the airways in horses. Some other countries license it for people, to open the airways. WADA, the world anti-doping agency, bans it in sport as a muscle-building drug.
Does clenbuterol burn fat?
Not in the only controlled study that measured it. Two weeks at 80 micrograms a day did not change fat mass at all in healthy men, while adding about 0.9 kg of lean mass. The heat people feel is real. The shaking and racing heart are real. The fat number did not move.
Is clenbuterol legal in the United States?
Not for people. The FDA has not approved it for people at any dose. The only US approval is for horses. Anything sold for people to take is unapproved, and WADA bans it in sport as a muscle-building drug.
What does clenbuterol do to the heart?
In the short term it makes the heart race. In poisonings it has caused low blood pressure, low potassium and blood signs of heart muscle damage. In animals it makes the heart muscle thicken. In people, two weeks of dosing did not change heart muscle mass. That is the only heart structure measurement anyone has published, and it is far too short to answer the question.
Why do athletes test positive from eating meat?
Because clenbuterol is used illegally in some countries to bulk up farm animals, and it builds up in the liver. The Spanish outbreak of 113 poisonings came from veal liver. Tainted meat is a proven route, which is also why it is an excuse nobody can disprove in doping cases.
Why do people cycle it two weeks on, two weeks off?
There is real science behind it, even though the schedule came from trial and error. In the human study, the receptor signal and the growth signal after it both faded over the two weeks. That is the receptor going numb. What no data supports is that cycling makes long-term use safe.