Clenbuterol is a controversial stimulant that has a variety of effects on the body. Although it may improve breathing, aid in weight loss, and muscle gain, it comes with serious risks. Popular with bodybuilders and others looking to trim fat and get in shape, its side effects are often overlooked. Read along to learn more about the health effects, side effects, risks, dosages, and user experiences of clenbuterol.

Disclaimer: By writing this post, we are not recommending this drug. 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. Please discuss your medications with your doctor.

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, clenbuterol may increase muscle mass and reduce fat [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].


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].

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].

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, 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) Burns Fat

In animals given doses far greater than those required to relax the airways, clenbuterol reduced fat mass more other fat burners, like ephedrine. This is due to increased metabolic energy (thermogenic effect) [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) Builds and Restores Muscles

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 and has potential in 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) Helps with 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. Clenbuterol is useful in maintaining muscles that are unused due to injury or disease [42, 43, 44, 45].

Clenbuterol treatment (6-18 months) improved the condition of 4 patients with a muscle-degrading disease (muscular dystrophy). The study concluded that clenbuterol may be useful in the early stages of this condition [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. Clenbuterol is useful at stopping muscle degradation in muscles with dysfunctional nerves [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].

Clenbuterol and Elderly

In the elderly, muscle loss and depleted protein stores can be the result of a poor diet or a disease. Rats depleted of proteins due to restricted diet showed a reversal of muscle wasting when treated with clenbuterol and a full diet. Therefore, clenbuterol may have future use in elderly individuals with degrading muscles [50, 2].

Clenbuterol and Cancer

Clenbuterol increased appetite and muscle weight in growing mice with tumors. This points to potential use in maintaining muscle mass in individuals suffering from chronic diseases, such as cancer [51].

Another study in rats with cancer showed that clenbuterol (with insulin and naproxen) encouraged weight gain and energy recovery [52].

5) Acts as a 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 [53, 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 [54].

6) May Improve Heart Function

7 patients with heart failure required device implantation (left ventricular assist device) to facilitate their recovery. High doses of clenbuterol were given for 5 to 46 weeks, and this prevented an expected decrease in the size of heart cells. It helped restore heart (ventricular) function and implantation of the device [55].

7) May Help with Urinary Urgency

Clenbuterol improved symptoms by 65% in a study of 30 women with urinary urgency after 6 weeks [56].

8) May Be Anti-Inflammatory

Rat studies concluded that clenbuterol may protect the brain and liver from inflammation-induced damage [57, 58].

This anti-inflammatory effect is also seen in horses and rats, in which white blood cell activity was reduced by clenbuterol [59, 60, 61, 62].

Studies in cells indicate that clenbuterol reduces the production of TNF-alpha, IL-6 and IL-1 beta [63, 64].

9) May Protect the Brain

Clenbuterol enhanced nerve cell activity in rats by increasing the activity of a nerve growth factor. This protects the brain from damage caused by an inadequate blood supply (and oxygen deficiency). Additionally, clenbuterol protects rat brains from damage through the same mechanism [R, 65].

Clenbuterol increased working memory in newly developing and aging rats and monkeys [66].

In rat brain cells, clenbuterol increased the production of kynurenic acid, a molecule which could have brain-protective activity [67, 68].

10) Reduces 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  [69, 70, 71, 72].

11) May Be an Antidepressant

In mice, clenbuterol increased tryptophan and serotonin levels, similar to many anti-depressant drugs. More studies would need to evaluate the potential of clenbuterol as an antidepressant in humans [73, 74].

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 [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].


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, 75, 76, 77, 78]:

  • 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 [79, 80, 81, 82, 78].


The long-term side effects of clenbuterol are largely unknown. Most are documented cases of athletes using clenbuterol for performance-enhancement [83].

Muscle Deformities

Although not seen in humans, clenbuterol use caused muscle deformities and fatigue in mice [84].


Clenbuterol acts on dopamine, the body’s reward system, and can be highly addictive [85].

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 [86, 87, 88].

One patient using clenbuterol (and solifenacin) developed severe and potentially fatal abnormal heartbeat (Torsade de pointes) [89].

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 [90].

In studies on rats and mice, clenbuterol caused anatomical and functional heart changes, including enlargement, which can have detrimental effects [17, 91, 92].

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 [93].

The combination of clenbuterol with other beta-2 receptor activators may be dangerous and increase the risk of serious side effects [94].

Clenbuterol is broken down by CYP enzymes (CYP1A1 and 1A2), which may affect the activity of other drugs that use the same detoxification pathway [95, 96].

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 [97].

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].

User Experiences & Abuse

Weight Loss

Many users found clenbuterol to be a “perfect weight loss pill” and reported good results with little to no negative side effects.

Users say that clenbuterol helped them burn a few more calories, but was not worth the side effects. Some users only take clenbuterol to get rid of the last few pounds for short periods of time.

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.

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