Clenbuterol Introduction

Clenbuterol (AKA Spiropent, Ventipulmin)
Chemical Name: (RS)-1-(4-Amino-3,5-dichlorophenyl)-2-(tert-butylamino)ethanol
Molecular Weight:
277.19 g/mol
Original Manufacturer:
Half Life:
36 – 48 hours
Detection Time:
4 – 5 days
Anabolic Rating:
Androgenic Rating:

Overview and History of Clenbuterol

Clenbuterol is not an anabolic steroid, but instead belongs to a certain class of compounds and drugs known as sympathomimetics (more commonly known as stimulants). It is specifically a sympathomimetic amine. Many other drugs belong to this ‘family’, some of which include: Caffeine, Albuterol, Ephedrine, Dextroamphetamine, Cocaine, and many others. This is a very broad drug category. All of these compounds, which include Caffeine, Albuterol, Clenbuterol, Ephedrine, Epinephrine, Norepinephrine, etc. are all related to one another and could be considered siblings or ‘cousins’ to each other, considering they are related in many ways and carry many similarities between one another. Clenbuterol in particular stimulates the sympathomimetic central nervous system in a few different ways. These nervous system stimulating effects work through the manner of receptors known as adrenoreceptors. These receptors are found in a plethora of different tissues throughout the body. Among many of the tissues these receptors are located, Clenbuterol acts upon these receptors and the result manifests in the form of different effects depending on which type of cells the receptors are located on. It is important to understand that there exist approximately 9 different adrenoreceptors throughout the body. As such, they are labeled as Alpha or Beta receptors with a series of numbers attributed to them for further labeling purposes. For example, there are Alpha-1, Alpha-2, Beta-1, Beta-2, etc. receptors located throughout the body. The different stimulants and sympathomimetic compounds differ in not only their individual action that they may have over their cousin compounds, but also in how they interact with these receptors. Its specifically well known for its interaction with the Beta-2 receptors, which is why its referred to as a Beta-2 receptor agonist. The relevance of this to Clenbuterol’s fat burning effects will be expanded upon shortly. Clen itself possesses a half-life of approximately 36 – 48 hours, which easily allow for single daily doses to be sufficient and no need for multiple daily doses.

Common Clenbuterol Uses
Clenbol 10Even though Clenbuterol is utilized first and foremost as a fat burning agent in athletic and entertainment circles, its original use in medicine was actually very different. All kinds of stimulants and sympathomimetics have been provided a medical purpose in the treatment of a broad spectrum of medical conditions, while many of them share similar attributes and can treat some of the same medical conditions as well. Clenbuterol specifically has been used a great deal to treat individuals suffering from asthma. This is the reason why its been the primary active ingredient in asthma inhalers (although for a long time now in North America, Albuterol is now the active ingredient in use in asthma inhalers – Clenbuterol is no longer used in North America for this purpose). The reason why its so useful in treating asthma is because, upon activating Beta-2 receptors in certain tissues (specifically, the cells lining the bronchial tube pathways), it will enable bronchial dilation (expanding of the airways) in the lungs, nose, and throat. This is one of the previously mentioned attributes that all sympathomimetics share with one another (bronchial dilation) to varying degrees. The body’s own naturally created neurotransmitters Epinephrine (Adrenaline) and Norepinephrine (Noradrenaline), which are also sympathomimetic stimulants, exhibit this effect as well but to a far greater extent. Other approved medical uses for Clen include: hypertension, cardiovascular shock/slowdown, arrhythmias, migraine headaches, allergic reactions and swelling, histamine reactions, and anaphylactic shock. It has been previously mentioned that Clenbuterol is a Beta-2 agonist because of its primary action on Beta-2 receptors in the body, but it must be clarified that Clenbuterol does exhibit activity on other receptor sites, but to a far lesser degree and as a result, far less significant. As an example, when Clenbuterol is compared to Ephedrine, we see that it acts primarily on Beta-2 receptors while Ephedrine has been shown to act on a plethora of Beta and Alpha receptors equally. For this particular scenario, the following analogy for ease of explanation could be used: Clenbuterol’s action of selectively activating Beta-2 receptors is the equivalent of having several nails sticking out of a wooden surface, and a hammer is used to hammer one specific nail on the head, while Ephedrine is the equivalent of using a larger sledgehammer to hit multiple nails on the head to drive them into the wood. Although not a perfect analogy, this explains Clenbuterol’s activity with a fair amount of accuracy.

The Activity of Clenbuterol In Tissues

Clenbuterol’s use as a bronchodilator for asthma sufferers has been explained above, including its medical application for other medical conditions. What is yet to be explained, and what the main focus here is, is its effect on fat metabolism. It is a very well-known fact that it has minor anabolic effects in muscle tissue. Some animal studies have been performed which have clearly demonstrated Clenbuterol’s anabolic activity in muscle tissue, but this has been found to be a very mild and minor effect. The truth of the matter is that the studies that have been conducted, as well as the anecdotal evidence from human users and bodybuilders, have demonstrated that Clenbuterol’s anabolic effects are very negligible at best. One study conducted on Clen observing its effects on skeletal muscle tissue in rats had concluded that Clenbuterol does exhibit minor anabolic effects in animals, but that a long duration of action appears to be required in order to induce these anabolic effects[1]. It would then be advisable to any potential user of Clenbuterol to understand that anyone seeking to use it purely for its supposed anabolic effects would more than likely result in disappointment and disappointing results. If used for the purpose of physique/performance enhancement, should be regarded first and foremost as a fat loss agent and should be used as such instead of an ‘anabolic’ compound (of which it does a very poor job). Further evidence for the negligible anabolic effects come from analysis of the many studies performed on animals in which the animal test subjects had been administered Clen at doses considered extreme and highly dangerous for a human. The fact that the majority of these studies have been performed on animals has raised the question as to whether or not Clen is actually anabolic at all in humans.

In any case, the selective Beta-2 action is what provides some significant fat loss attributes. It is through the activation of these B2 receptors located on the surface of fat cells where Clenbuterol can directly activate and initiate fat metabolism, and initiate the process of triglyceride breakdown into free fatty acids. B2 receptors on fat cells are known to be primary influences in the role of fat metabolism, and perhaps to a greater extent than other Beta or Alpha receptors located on fat cells as well. It is the enhanced rate of fat metabolism provided by Clen that is the reason why Clen buterol in the past had frequently been administered to cattle in conjunction with various anabolic steroids. The purpose of this practice was to ensure the largest amount of lean fat-free meat for slaughter and subsequently provided as food. One important factor to note for any potential user is that Clenbuterol’s fat loss effects may not be as effective in every individual. Some may respond more favorably to the fat loss effects, while others may find that they respond better in terms of fat loss with other compounds such as Ephedrine (perhaps due to Ephedrine’s greater activity with more receptor types). The point here is to remember that although Clenbuterol should elicit favorable fat loss in most individuals, non-responders (or people who respond very poorly) do exist.

One final note on Clenbuterol’s Beta-2 agonistic activity is that of Beta-2 receptor downregulation. It is through regular daily use of Clen that Beta-2 receptors can and will down regulate in response to Clenbuterol’s activity on those receptors[2]. What results is a continually diminished fat loss effect over time, which must be remedied one of two different ways. The first method would be time off from the drug (2 weeks minimum or longer). However, studies have found that the use of Ketotifen (Ketotifen Fumarate, an anti-histamine drug) actively up-regulates these Beta-2 receptors[3]. This will be further expanded upon, but diminishing fat loss resulting from the continued use of Clenbuterol tends to appear around the 2nd or 3rd week, and varies depending on the individual. If not using Ketotifen, a 2 week break is the typical protocol in order to allow the body to naturally up-regulate its receptor sites. Finally, what must be noted is the rumor of Benadryl (Diphenhydramine Hydrochloride) use for receptor up regulation. This is known as a false rumor, and it is not advised that any individual utilize Benadryl with the hopes of up regulating Beta-2 receptors to increase fat loss. There is no harm in the use of Benadryl with Clen, but one would be wasting their time and money engaging in this practice, and it is important to be clear that the use of Benadryl for this purpose was found to be a baseless rumor, and an ineffective practice.




[1]Anabolic effects of clenbuterol on skeletal muscle are mediated by beta 2-adrenoceptor activation. Choo JJ, Horan MA, Little RA, Rothwell NJ. Department of Physiological Sciences, University of Manchester Medical School, United Kingdom. Am J Physiol. 1992 Jul;263(1 Pt 1):E50-6.

[2] Effect of dietary clenbuterol and cimaterol on muscle composition, beta-adrenergic and androgen receptor concentrations in broiler chickens. Schiavone A, Tarantola M, Perona G, Pagliasso S, Badino P, Odore R, Cuniberti B, Lussiana C. J Anim Physiol Anim Nutr (Berl). 2004 Apr;88(3-4):94-100.

[3]Effects of ketotifen and clenbuterol on beta-adrenergic receptor functions of lymphocytes and on plasma TXB-2 levels of asthmatic patients. Huszar E, Herjavecz I, Boszormenyi-Nagy G, Slapke J, Schreiber J, Debreczeni LA. Z Erkr Atmungsorgane. 1990;175(3):141-6.