Clenbuterol found its way into gyms as a "fat burner", although it was created as a bronchodilator to treat asthma, and in many countries it is generally only allowed in veterinary medicine. Its side effects are not rare exceptions - they directly result from the mechanism of action of the drug. The editors have collected exactly what toxicologists, cardiologists and clinical pharmacologists describe, and explain why these reactions occur even in healthy young people.
Why does Clenbuterol have so many side effects
Clenbuterol is a long-acting beta-2-adrenoceptor agonist. Beta-2 receptors are not only located in the bronchi: they are found in skeletal muscles, blood vessels, liver, adipose tissue, pancreas and heart. Therefore, a drug that "turns on" these receptors throughout the body inevitably affects dozens of systems at the same time. What is called a side effect in pharmacology is often just a continuation of the main action in the "wrong" tissue.
The second reason is that the selectivity of clenbuterol for beta-2 receptors is relative. In high concentrations, it also stimulates beta-1 receptors of the heart, which are responsible for the frequency and force of contractions. In addition, even pure beta-2 stimulation causes vasodilation, a fall in peripheral resistance, and a reflex acceleration of the pulse.
The third reason is the long half-life. According to pharmacokinetic studies, it is more than a day, that is, much more than that of salbutamol. The drug accumulates in the body, and if a person takes it every day, the concentration increases for several days in a row. Because of this, side effects persist for a long time and do not disappear immediately after skipping a dose.
Finally, in nonmedical use, doses reported by poison center patients often exceed those once used to treat bronchospasm. Added to this is the unknown content of "sports" products: in several clinical reports, blood tests showed concentrations that the person could not explain with the stated dose.
Therefore, the side effects of Clenbuterol are not "individual intolerance". These are predictable pharmacological reactions that increase with increasing dose and duration of administration.
The most frequent reactions: from tremors to insomnia
The most famous effect is tremor of the hands. Beta-2 receptors of skeletal muscles change the speed of fiber contraction, and the hands begin to tremble slightly. In people who use the drug not as prescribed, tremors often appear in the first days and can interfere with precise movements, writing, and driving.
The second typical symptom is an accelerated heartbeat and a feeling of "thumping" in the chest. In a retrospective analysis of referrals to US poison control centers (Spiller et al., 2013), tachycardia was the most common clinical presentation among cases associated with weight loss and bodybuilding. A similar picture was described by the Australian center of New South Wales (Brett et al., 2014).
Insomnia, anxiety, internal tension and sweating — a consequence of the general sympathomimetic action. A person may feel "overexcited", have trouble falling asleep, wake up in the middle of the night with a palpitation. Some users perceive this as a "sign that the drug is working", although in fact it is a manifestation of stress on the nervous system.
Painful spasms of muscles, especially calves and feet, are quite characteristic. They are associated with a decrease in the level of potassium and with a direct effect on skeletal muscles. Headache, nausea, dry mouth, increased blood pressure or, on the contrary, its fluctuations, also often appear.
| Reaction | Probable mechanism | Comment |
|---|---|---|
| Tremor | Stimulation of beta-2 receptors of skeletal muscles | One of the first and most frequent manifestations |
| Tachycardia, palpitations | Vasodilatation, reflex and direct stimulation of the heart | The leading reason for referrals to toxicologists |
| Insomnia, anxiety | Sympathomimetic action on the central nervous system | Intensifies when combined with caffeine |
| Muscle cramps | Hypokalemia, influence on muscle metabolism | Often in calves and feet |
| Headache, sweating | Vascular and autonomic effects | Nonspecific, but frequent |
Metabolic disorders: potassium, glucose, lactate
One of the least obvious but most important actions of clenbuterol is potassium redistribution. Stimulation of beta-2 receptors activates the sodium-potassium pump, and potassium from the blood "enters" the cells. The total supply of potassium in the body may not change, but its level in the plasma drops. It is the concentration in the plasma that determines the electrical stability of the heart.
Hypokalemia was among the most frequent laboratory findings in toxicology center reports. A clinical case described by Hoffman et al (2001) demonstrated a combination of sustained tachycardia, hypokalemia, and hypophosphatemia with laboratory confirmation of clenbuterol in the blood. Low potassium provokes muscle weakness, cramps and rhythm disturbances.
Clenbuterol also increases glucose levels. Beta-2 stimulation increases the breakdown of glycogen in the liver and muscles and at the same time stimulates the release of insulin. In a healthy person, this is usually manifested by moderate hyperglycemia, in a person with impaired glucose tolerance, by more pronounced fluctuations.
Another finding is an increase in lactate in the blood. Enhanced glycolysis in muscles leads to the accumulation of lactic acid even in a state of rest. Some reports also describe hypomagnesemia and hypophosphatemia, which exacerbate muscle symptoms and the risk of arrhythmias.
- Potassium: redistribution into cells lowers blood levels, increasing the risk of arrhythmias, weakness and muscle cramps.
- Glucose: increased by glycogenolysis — important for people with diabetes.
- Lactate: Elevated due to increased glycolysis — may mimic other conditions.
- Phosphate and magnesium: May decrease, exacerbating muscle weakness.
These changes explain why clenbuterol is viewed in the medical literature as a systemic toxicant and not simply a "strong stimulant". Even if a person feels normal, laboratory values may already be outside the safe range.
Serious complications and poisoning
The most dangerous complications concern the heart. Cases of supraventricular tachycardia, atrial fibrillation, ventricular extrasystoles and even myocardial infarction in young people without coronary disease have been described. For example, Kierzkowska et al (2005) reported a heart attack in a 17-year-old bodybuilder taking clenbuterol. We discuss cardiac risks in more detail in a separate article.
In experiments on rats, Burniston and colleagues (2002) showed that clenbuterol in sufficient doses causes cardiomyocyte death and damage to soleus muscle fibers. Direct transfer of these results to humans is incorrect, but they provide a biological explanation for clinical cases of myocardial damage.
A separate layer of data — mass food poisoning. In 1990, an outbreak of poisoning was described in Spain after consumption of the liver of animals illegally fattened with clenbuterol (Martínez-Navarro, 1990). People complained of tremors, palpitations, nervousness and headaches. Similar episodes were later reported in other countries, which became one of the reasons for banning beta-agonists in animal husbandry.
Toxicologists note that the symptoms of clenbuterol poisoning can last much longer than after an overdose of short-acting beta-agonists, precisely because of the long half-life. Inpatient treatment includes ECG monitoring, electrolyte correction, and, as indicated, the prescription of beta-blockers under physician supervision.
It is also important that the risk increases when combined with other substances: caffeine, synephrine, yohimbine, thyroid hormones, stimulants. In such combinations, the effects on the heart are summed up, and a person can rarely predict how his body will react.
Who is in the risk group and how the condition is assessed
People with any heart rhythm disorders, hypertension, structural heart disease, hyperthyroidism, diabetes and epilepsy are most vulnerable. However, clinical cases show that severe reactions occur even in young athletes who considered themselves completely healthy. Hidden cardiomyopathy or long QT interval syndrome may first appear precisely during a stimulant.
Women who use clenbuterol for weight loss often have a lower body weight, so the same amount of the substance creates a higher concentration in them. In addition, the strict diets common in this group themselves reduce potassium and magnesium levels, increasing the risks.
When consulting a doctor with suspected toxic effects of clenbuterol, ECG, heart rate, blood pressure, level of potassium, magnesium, phosphate, glucose, lactate are usually evaluated, and in the presence of chest pain, markers of myocardial damage (troponin). It is important for a person to honestly tell the doctor what he has been taking - the correct treatment depends on this.
Warning signs that require emergency care: chest pain or pressure, an irregular heartbeat, dizziness or fainting, shortness of breath at rest, severe muscle weakness, pulse that does not slow down at rest. It is dangerous to wait for things to "go away by themselves" in such situations.
Editorial conclusions
Side effects of clenbuterol are embedded in the very mechanism of its action: stimulation of beta-adrenoceptors in the heart, muscles, blood vessels and liver inevitably causes tremors, palpitations, insomnia and metabolic shifts.
The most serious risks are hypokalemia, arrhythmias, and myocardial damage. They are described in toxicology center reports and clinical cases, including in young people without known heart disease. The long half-life makes the reactions long-lasting.
The actual benefit for reducing fat mass in humans is poorly confirmed, and the risk-benefit ratio, according to the editors, is clearly not in favor of the drug.
If the topic is close to you, we advise you to read our materials "Clenbuterol and the heart: risks for the cardiovascular system", "Clenbuterol contraindications" and "Clenbuterol myths".
References
- Spiller HA, James KJ, Scholzen S, Borys DJ. A descriptive study of adverse events from clenbuterol misuse and abuse for weight loss and bodybuilding. Subst Abus. 2013;34(3):306–312.
- Brett J, Dawson AH, Brown JA. Clenbuterol toxicity: a NSW poisons centre experience. Med J Aust. 2014;200(4):219–221.
- Hoffman RJ, Hoffman RS, Freyberg CL, et al. Clenbuterol ingestion causing prolonged tachycardia, hypokalemia, and hypophosphatemia with confirmation by quantitative levels. J Toxicol Clin Toxicol. 2001;39(4):339–344.
- Kierzkowska B, Stańczyk J, Kasprzak JD. Myocardial infarction in a 17-year-old body builder using clenbuterol. Circ J. 2005;69(9):1144–1146.
- Burniston JG, Ng Y, Clark WA, et al. Myotoxic effects of clenbuterol in the rat heart and soleus muscle. J Appl Physiol. 2002;93(5):1824–1832.
- Martínez-Navarro JF. Food poisoning related to consumption of illicit beta-agonist in liver. Lancet. 1990;336(8726):1311.
- Pope HG Jr, Wood RI, Rogol A, et al. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. Endocr Rev. 2014;35(3):341–375.
