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Clenbuterol: What is it Used For + Side Effects & Dangers

Written by Puya Yazdi, MD | Reviewed by Ana Aleksic, MSc (Pharmacy) | Last updated:
Jonathan Ritter
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology) | Written by Puya Yazdi, MD | Reviewed by Ana Aleksic, MSc (Pharmacy) | Last updated:

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Clenbuterol is a controversial stimulant that has a variety of effects on the body. Although it may improve breathing and is used off-label by bodybuilders looking to lose weight and gain muscle, it comes with serious risks. Read along to learn more about the effects, side effects, and dangers of clenbuterol.

Disclaimer: We strongly recommend against taking clenbuterol for any reason. Some of our readers who were already taking the drug requested that we commission a post on it, and we are simply providing information that is available in the clinical and scientific literature.

The FDA has not approved clenbuterol for use in humans. Veterinarians administer clenbuterol to clear airway obstruction in horses, and outside of the United States, it is occasionally used to treat chronic obstructive pulmonary disease (COPD). It is important to discuss your medications with your doctor, and to never take a drug without the prescription and supervision of a licensed medical professional.

What is Clenbuterol?

Clenbuterol is a synthetic stimulant (not a steroid), with similar short-term effects to other stimulants (such as amphetamines or ephedrine). These include increased heart rate, blood pressure, breathing, and metabolism. In fact, it was first developed to promote mass gain in animals. Long-term use in animals has been associated with increased muscle mass and reduced fat, but no clinical studies have found similar results in humans [1, 2].

Clenbuterol has multiple trade names, including Spiropent, Dilaterol, Ventolase, and NAB 365 [3, 4].

Clenbuterol Mechanism of Action

Clenbuterol activates the fight-or-flight system in humans (“sympathomimetic”). Its effects include [5, 6]:

  • Widening the airways to improve breathing and increase airflow to the lungs (bronchodilator).
  • Increasing heart rate, and blood pressure.
  • Sends more blood to the muscles to prepare for physical activity.
  • Increasing muscle mass (catabolic) and burning fat for more energy.

It also increases sweating and short-term toxicity presents similarly to other stimulants (amphetamine or ephedrine) [7].

Although diverse, all the effects are caused by activating beta-2 receptors found throughout the body [1].

Muscle-Building

Clenbuterol induces growth, prevents breakdown, and speeds recovery of muscles. It stimulates protein production within cells, which eventually increases muscle mass (via beta-2 activation) [8, 9].

Muscle gain induced by clenbuterol occurs due to increases in overall protein levels and creatine kinase. Creatine kinase increases muscle cell joining (myoblast fusion), a key process in the formation and repair of muscles [10, 8].

Clenbuterol also prevents muscle and protein breakdown after nerve damage, allowing the muscles to rebuild [11, 9, 12].

These findings come from animal studies only; no clinical studies have found similar results in humans.

Genetics and Clenbuterol

The effects of clenbuterol may vary depending on an individual’s genetics. Genetic variations in the beta-2 receptor in different individuals may change the body’s response to clenbuterol [1].

In mice, clenbuterol caused changes in the activated genes for the beta-2 receptor, proteins involved in fat metabolism, and several muscle proteins. It may cause similar changes in humans too, which can have varying effects [13].

In a study on pigs, clenbuterol activated enhanced the production of many genes (in fat cells) involved in fat metabolism, decreasing fat stores and the size of fat cells [14].

A study on tissues showed that clenbuterol increases levels of a genetic regulator (histone demethylase JHDM2a) by activating beta-2 receptors (via protein kinase A pathway). This regulator controls metabolic (and other related) genes, which ultimately decreases the size of fat cells [15].

Once again, it is important to remember that no human studies have found links between clenbuterol and the activity of any genes. This branch of research is highly speculative, and human studies will be required to determine what connection – if any – exists between clenbuterol and human genetics.

Is Clenbuterol Legal?

Clenbuterol import for sale or human use is banned in most countries and is not approved for human consumption by the FDA. Despite this, clenbuterol use is still high amongst bodybuilders and individuals seeking rapid weight loss [16, 17].

The World Doping Agency (WADA) and the International Olympic Committee (IOC) have banned clenbuterol as a performance-enhancing drug [18, 19, 17].

However, clenbuterol is still used in some countries for treating asthma [1].

Clenbuterol is illegally used to increase growth in livestock [20, 21].

Contaminated Meat

Clenbuterol was banned for livestock in most countries since large amounts of the drug can enter the body when eating the contaminated meat. That appeared to be the case for Spanish cyclist, Alberto Contador, who failed a Tour de France drug test in 2010 [18, 19, 17].

There are a number of other reports of clenbuterol-contaminated meats in different countries:

  • 140 people in Spain hospitalized after eating meat (veal) tainted by clenbuterol [22].
  • 336 people in China poisoned after eating contaminated pork [23].
  • 50 cases of acute food poisoning in Portugal reported from lamb and beef contamination [24].
  • 62 people in Italy reported affected by meat (veal) contaminated with clenbuterol [25].

How Long Does it Stay in Your System?

Clenbuterol is detectable in urine after multiple-doses. It can also be found in hair samples one month after ingestion [26, 27, 28].

Uses of Clenbuterol

1) Improves Breathing

Clenbuterol treats asthma by improving obstructed breathing. It relaxes and widens blood vessels. This increases the volume of oxygen in the blood and increases the body’s capacity to use oxygen for energy (aerobic respiration) [3].

In two different studies (double-blind randomized controlled trials), clenbuterol significantly improved breathing in asthmatic patients. Clenbuterol had a more rapid-onset and long-lasting effect in comparison to salbutamol [3, 29].

In addition, varying doses of clenbuterol were administered to 12 children with severe asthma. This significantly expanded their airways in comparison to placebo [30].

Some countries still use clenbuterol in humans as a decongestant and bronchodilator. However, there are many safer alternatives [31].

Clenbuterol is also often used to treat breathing problems in horses [32].

2) Fat Burning

In animals given doses far greater than those required to relax the airways, clenbuterol reduced fat mass more than other fat burners, like ephedrine. This is due to increased metabolic energy (thermogenic effect). However, the doses tested on these animals would be lethal in humans [33, 34].

Clenbuterol increases heart rate and blood pressure. This leads to an increase in metabolic activity and body temperature, which in turn burns more fat and stored carbohydrates (glycogen) [35].

Athletes often illegally use clenbuterol as a fat-loss drug during cutting cycles. 19 people with chronic heart failure treated with clenbuterol (randomized controlled trial) had a significant increase in lean muscle and lean-to-fat ratio. These effects are also seen in rats and horses [33, 36, 37].

3) Muscle Growth and Healing

Clenbuterol boosts the recovery of patients from surgery. 20 healthy male patients (double-blind randomized controlled trial) were treated with clenbuterol for one month after surgery (knee). Clenbuterol caused more rapid rehabilitation of muscle strength. Though these results are promising, the study was very small, and further research is required into clenbuterol’s potential for treating muscle-wasting conditions [38].

Clenbuterol induces muscle growth by increasing the production of protein and decreasing muscle loss. Clenbuterol used on rats led to a marked increase in muscle mass and protein production rates. Additionally, muscle fibers became thicker in pigs using clenbuterol [39, 15, 14].

Although, clenbuterol did not increase protein production in one study in rats. Instead, muscle-building was due to reduced protein breakdown [40].

A study in rats suggests that the effect of clenbuterol is muscle-specific with fast-twitch muscles growing more rapidly and degrading less. Fast-twitch muscles are used in powerful, quick bursts of activity [41].

Though now illegal in most countries, the meat industry has used clenbuterol to increase lean body mass (muscle) in animals for years [31].

4) Muscle-Degrading Conditions

16 patients with a muscle-degrading condition (muscular atrophy) were treated with clenbuterol. This significantly improved their mobility and reduced worsening of the condition. According to this one small study, clenbuterol could have a potential use in maintaining muscles that are unused due to injury or disease. Note, however, that clinical studies never followed up on this potential due to the dangers of clenbuterol [42, 43, 44, 45].

Clenbuterol treatment (6-18 months) improved the condition of four patients with a muscle-degrading disease (muscular dystrophy) [46].

Clenbuterol and Nerve Damage

71 patients (double-blind randomized controlled trial) with nerve damage (brachial plexus injury) that leads to muscle degradation were treated with a placebo or clenbuterol for 3 months. Those treated with clenbuterol had improvements in their muscle-related condition [47, 48].

Clenbuterol and Burns

Muscle degradation may occur as a result of a burn injury. Clenbuterol effectively reversed muscle degradation caused by tissue burns in rats [34, 49].

5) Stimulant

Athletes use clenbuterol as an energy enhancer. This is because clenbuterol acts similar to adrenaline in the body (activation of the sympathetic nervous system). It’s considered a performance-enhancing drug by most sporting authorities [50, 19].

As a stimulant, clenbuterol temporarily increases alertness, attention, and energy. This may make workouts more pleasurable and invigorating. Additionally, users may experience a suppressed appetite and improved motivation, competitiveness, and aggression while working out [51].

6) Urinary Urgency

Clenbuterol improved symptoms by 65% in a study of 30 women with urinary urgency after 6 weeks. Note that this was a very small study conducted in 1986, and it was abandoned due to the apparent dangers of the drug [52].

7) Labour Complications in Animals

Due to its muscle relaxant effects, clenbuterol is used in livestock to reduce labor complications. This indication was never researched in humans due to the risks clenbuterol carries. In cattle, clenbuterol and corrected birth-complications, reducing the need for anesthesia. But even in animals, this class of drugs can cause adverse effects, such as an increase in heart rate, bleeding, and birth defects [53, 54, 55, 56].

Dosage of Clenbuterol

Previously used at low doses as an effective bronchodilator, clenbuterol use is now limited to veterinarians and researchers. As such, there are no clear clinical dosage guidelines to follow.

Websites and blogs selling clenbuterol may give additional information on dosing, which can be dangerous due to the lack of research and serious risks. Remember that clenbuterol is only approved for use in horses and is considered too dangerous to be used in humans [32].

Side Effects and Interactions of Clenbuterol

Clenbuterol use can be dangerous, with many possible short- and long-term side effects. Clenbuterol is extremely potent – very small doses are needed to achieve the effects. Unfortunately, this can lead to more overuse, overdose, and accidental poisoning [24, 25].

Complications mainly arise from athletes abusing clenbuterol for performance enhancement. There have also been cases of clenbuterol poisoning from food contamination in Portugal and Italy [19, 24, 25].

Short-term

Since clenbuterol is a stimulant, short-term side effects are similar to other stimulants such as coffee and amphetamines. These can largely vary depending on the individual, especially if some level of tolerance is achieved. The following are common short-term side effects, especially in first-time users [1, 57, 58, 59, 60]:

  • Increased blood pressure, heart rate, and palpitations
  • Insomnia
  • Dry mouth
  • Vomiting and nausea
  • Tremors (shakiness, especially marked in hands)
  • Anxiety, Panic attacks, nervousness, and restlessness
  • Headaches
  • Breathing difficulties and rapid breathing
  • Intense sweating
  • Muscle cramps and spasms (due to dehydration or decreased potassium)
  • Dizziness
  • Increased body temperature
  • Manic symptoms (heightened energy)

Headaches, nausea, and muscle cramps are most common after intense training. Clenbuterol also depleted taurine, which protects the heart, and potassium, which is essential for muscle movements [61, 62, 63, 64, 60].

Long-term

The long-term side effects of clenbuterol are largely unknown. Most are documented cases of athletes using clenbuterol for performance-enhancement [65].

Muscle Deformities

Although not seen in humans, clenbuterol use caused muscle deformities and fatigue in mice [66].

Addiction

Clenbuterol acts on dopamine, the body’s reward system, and can be highly addictive [67].

Heart Attack

Clenbuterol can increase heart rate (tachycardia), as well as cause abnormal heartbeats, palpitations, and chest pain. This increases the risk of a heart attack, especially in individuals who are already susceptible [68, 69, 70].

One patient using clenbuterol (and solifenacin) developed severe and potentially fatal abnormal heartbeat (Torsade de pointes) [71].

In horses, clenbuterol should be avoided in those with existing heart conditions or high blood pressure. This means that clenbuterol may be especially dangerous for people with heart disease [72].

In studies on rats and mice, clenbuterol caused anatomical and functional heart changes, including enlargement, which can have detrimental effects [17, 73, 74].

Drugs Interactions of Clenbuterol

Many people are taking clenbuterol illegally and unsupervised. Physicians unaware of their patients’ clenbuterol use may prescribe medications that interact with this drug [75].

The combination of clenbuterol with other beta-2 receptor activators may be dangerous and increase the risk of serious side effects [76].

Clenbuterol is broken down by CYP enzymes (CYP1A1 and 1A2), which may affect the activity of other drugs that use the same detoxification pathway [77, 78].

Limitations and Caveats

Available evidence restricts clenbuterol use in humans. Only a few older double-blind studies or multiple cohort studies exist. However, there are many veterinary studies, some of which may have implications for humans [79].

Additionally, when a low-cost drug is discovered to be dangerous in humans, it’s still often used in animals. This could be because it’s not as dangerous for the animals, or the side effects are less acceptable in humans. That is the case for clenbuterol, which is why so much research is available on animals and not humans.

There are other drugs that are as effective as clenbuterol in humans, with fewer side effects. This adds to the lack of clinical research on clenbuterol, where the benefit to risk ratio is too low for the research to be worth it [33].

Clinical research on clenbuterol was abandoned due to unacceptably high risk in humans. Animals (horses, in particular) can tolerate the types of doses required for positive effects on muscle and fat—even adjusted for weight, similar doses of clenbuterol would kill a human being.

User Experiences & Abuse

Weight Loss

Users say that clenbuterol helped them burn a few more calories, but was not worth the side effects.

Many people noticed tolerance after more than 2 weeks, as the body gets used to its effects. Bodybuilders abuse it in cycles.

One new user said that her body temperature rapidly increased, felt hot-headed and anxious for at least 30 minutes.

Performance-Enhancement Abuse

Athletes and gym users abuse clenbuterol as a performance-enhancer to increase their energy levels, weight loss, endurance, and decreased appetite.

Muscle Shaping

Bodybuilders abuse clenbuterol in a cutting cycle to get extra shredded for vanity purposes or for competition.

One bodybuilder achieved weight loss but experienced severe muscle cramps in toes, fingers, legs, and jaw.

Takeaway

Clenbuterol is an illegal stimulant that some bodybuilders believe can help them lose weight and gain muscle. However, even in its early clinical trials investigating its effect on the airway, clenbuterol was found to be too dangerous to develop for human use. The doses required for this drug to affect muscle growth would very likely kill the human user.

Clenbuterol’s myriad side effects include increased blood pressure, vomiting, and insomnia. In the long term, it can cause addiction, muscle deformities, and heart attack. We recommend strongly against using clenbuterol for any purpose.

About the Author

Puya Yazdi

Puya Yazdi

MD
Dr. Puya Yazdi is a physician-scientist with 14+ years of experience in clinical medicine, life sciences, biotechnology, and nutraceuticals.
As a physician-scientist with expertise in genomics, biotechnology, and nutraceuticals, he has made it his mission to bring precision medicine to the bedside and help transform healthcare in the 21st century.He received his undergraduate education at the University of California at Irvine, a Medical Doctorate from the University of Southern California, and was a Resident Physician at Stanford University. He then proceeded to serve as a Clinical Fellow of The California Institute of Regenerative Medicine at The University of California at Irvine, where he conducted research of stem cells, epigenetics, and genomics. He was also a Medical Director for Cyvex Nutrition before serving as president of Systomic Health, a biotechnology consulting agency, where he served as an expert on genomics and other high-throughput technologies. His previous clients include Allergan, Caladrius Biosciences, and Omega Protein. He has a history of peer-reviewed publications, intellectual property discoveries (patents, etc.), clinical trial design, and a thorough knowledge of the regulatory landscape in biotechnology.He is leading our entire scientific and medical team in order to ensure accuracy and scientific validity of our content and products.

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