Understanding Clenbuterol


by Mike Arnold

When clenbuterol (clen for short) first hit the bodybuilding scene in the 1980’s, it was viewed in much the same way as most new performance enhancers—as the next best thing. Surrounded by an aura of mystery, excitement, and apprehension, in combination with a healthy dose of hype, it was only a matter of time before some crazy stories began to circulate around the community regarding its proficiency as an ergogenic aide. The general consensus was that this new wonder drug could melt off fat in record time, while simultaneously providing muscle building benefits on par with AAS.

As we entered the 90’s, the drug was still widely inaccessible to most (remember, this was the pre-internet days) and reliable, detailed information on PED’s remained scarce. At around this time we also started seeing the major bodybuilding publications paint clenbuterol as some kind of death drug in what could only be seen as an obvious attempt to turn BB’rs away from its growing use. While I don’t doubt that the intentions of the authors were well-meaning, this only served to further muddy the waters regarding our understanding of the drug. Still, being risk takers at heart, this didn’t dissuade BB’rs from continued self-experimentation.

As real-world experience continued to accumulate and with the internet on the horizon, things began to change. In time, the scare tactics faded away and were replaced with useful information, while exaggerations diminished and the drug’s true benefits were more clearly defined. At this point, our understanding of clenbuterol and its applications as a performance enhancer—or should I say “appearance enhancer”—are solid, being rooted in extensive clinical research and a massive amount of anecdotal evidence. However, for those of you who are not quite up to speed on this subject, the following article may help you gain a better understanding of this commonly used PED, while teaching you how to properly implement it into your program.

What is Clenbuterol?

Technically, clenbuterol is what we call a Beta-2 adrenergic agonist. It belongs to the class of drugs known as sympathomimetics and works by latching onto and activating the Beta-2 receptor, which is found in various tissues such as muscle and fat. Depending on which receptors are activated, it can produce a wide range of effects throughout the body.
Possessing numerous applications in both humans and animals, it is routinely used by asthmatics as a bronchodilator and more recently it has been employed as a cardio-protectant in those suffering from congestive heart failure (in combination with a B1 AR blocker). In animals, it is used world-wide as a repartitioning agent in livestock as a cost-effective method of increasing meat yield, as well as a performance enhancer in race horses, and less frequently as a labor suppressant in some animals.

Clenbuterol as a growth Agent

There has been a lot of misinformation put forth over the years regarding Clen’s viability as a growth promoting agent, most of which has come about as a result of extrapolation (applying information from animal research to humans). The problem here is that animal studies, due to physiological differences between the species, are often poor predictors of human response, so using them for anything other than speculation is irresponsible. In order to come to any type of reliable conclusion, human research is mandatory. This necessity is clearly demonstrated when comparing the documented effects of clenbuterol in animal and human studies.

When it comes to tolerance, animals definitely have the upper-hand, being capable of using doses 200-400X the normal human limit without experiencing ill effects. At these dosages, dramatic body recomping effects are noticed within a very short period of time, with one study revealing a whopping 20% increase in lean mass and a full 20% decrease in bodyfat in less than 10 days! That is a near unbelievable change in body composition, especially given the fact that these animals did not undergo any type of resistance training or other hormone therapy.

Unfortunately, the results aren’t nearly as impressive in humans, with some studies showing very modest improvements and others nothing at all. Whether this is due to the much smaller dose employed, variances in physiological response, or both, the fact remains that Clen does not work well as a growth enhancer in humans. Although its weak anabolic nature makes it unsuitable as a mass-builder, its muscle sparring effects can be useful during a diet. Unlike most other fat loss drugs, which tend to have deleterious effects on muscle maintenance, clen is one of the few that actually has a positive effect in this area, allowing the user to strip away bodyfat while reducing the risk of muscle loss. This can be especially valuable towards the end of a contest diet, when low caloric intake makes holding onto muscle tissue difficult. This dual effect qualifies Clen as an effective repartitioning agent.

Also of interest is Clen’s ability to increase Type II muscle fiber density (an increased ratio of Type II to Type I muscle fibers), but don’t get your hopes up, as this effect has only been observed in animals and again, is likely due to substantial differences in dosing. With the majority of clen’s growth and muscle fiber altering benefits only occurring at these elevated doses, why haven’t they been studied in humans? The answer is simple and is directly related to clen’s effect on beta 1 receptors. Although clen is generally considered to be selective in its activation of the Beta 2 receptor, it still has a pronounced effect on the Beta 1 receptor, which controls things such as heart rate and blood pressure. When Beta 1 receptors become overstimulated, heart rate can rise to dangerous levels, causing myocardial infarction (heart attack) and death.

We can potentially inhibit Clen’s effect on Beta 1 receptors by co-administering a Beta 1 blocker (ex. metropolol), which in theory, should allow us to take advantage of the benefits associated with higher dosages, while avoiding the negative effects on heart rate and blood pressure. However, this is not recommended. With clenbuterol and Beta 1 blockers possessing different half-lives, even the slightest miscalculation in timing could potentially result in the re-stimulation of beta 1 receptors, followed by all the side effects normally encountered with excessive clen dosages. For this reason, such experimentation is best left for medical researchers.

Clen as a fat-loss Agent

This is where Clenbuterol really shines. Used for decades in the bodybuilding and fitness communities, clen has built a reputation as one of the most effective fat loss drugs available and with good reason. Functioning as a both a thermogenic and lipolytic agent, Clen stimulates fat loss through not one, but two potent, independent mechanisms. This allows the user to experience fat loss at a greatly accelerated rate.

As a lipolytic agent, Clen potentiates fat burning by attaching to beta 2 receptors within adipose tissue, which initiates the breakdown and release of stored triglycerides into the bloodstream, making them more readily available for use as energy. As a thermogenic, clen also provides a direct fat burning effect by increasing the rate at which fatty acids are oxidized within the mitochondria. This leads to one of clen’s defining characteristic effects—an increase in body temperature. As mentioned earlier, beta 2 agonists possess moderate muscle sparring effects, differentiating them from fat burners such as T3, which promote fat burning through indiscriminate calorie burning (both fat & muscle). In my opinion, Clenbuterol, along with growth hormone, are the best fat loss drugs for BB’rs, especially during pre-contest prep, as they help accomplish the BBr’s two primary goals—the maintenance of muscle tissue and the elimination of bodyfat.

Administration & Dosing

Over the years, clenbuterol has been subject to considerable disagreement in terms of administration. 2 days ON/ 2 days OFF, 1 week ON/ 1 week OFF, 2 weeks ON/ 2 week OFF, 3 week ON/ 3 weeks OFF, and just about every other dosing scheme you can think of has been recommended over the last 20 years. Most of the conflict originates from an attempt to optimize its fat burning effects and minimize down-regulation, but before we address this issue, let’s first take a look at what down-regulation is and why it should be avoided.

Down-regulation is the process by which a cell decreases the quantity of a cellular component in response to an external variable. Using clen as an example, down-regulation occurs when the number of beta 2 receptors diminishes as a result of continued clenbuterol administration. In laymen’s terms, using clen for too long will cause the cell to stop responding to the drug. The key to successful clenbuterol administration, at least in terms of maintaining efficacy, is to use the drug only as long as its fat burning effects are maintained. While this limits the amount of time we can use the drug, we can always go back on after beta 2 receptors have up-regulated, which occurs naturally with sufficient off-time.

Let’s look at some of the different dosing schedules and see which is best. For a time, the 2 day ON/ 2 day OFF program was popular, but it fails to take in account the drug’s long duration of action. With a half-life of roughly 35 hours, this dosing schedule never allows the drug to clear the system, leading to sustained down-regulation after just a few weeks of use. Obviously, anyone who recommends this program lacks a basic understanding of the drug’s pharmacokinetic profile.

While down-regulation begins to take place after about 2 weeks, its effectiveness can be partially maintained for a longer period of time, as full down-regulation does not occur for several weeks. Still, unless the user has a rapidly approaching deadline which requires him to be in shape (ex. an impending contest), most users will be better off sticking with the traditional 2 weeks ON/ 2 weeks OFF schedule, as it provides the best balance of effectiveness to down-regulation.

When it comes to dosing frequency, clen has traditionally been administered 1-2X daily. Being a nervous system stimulant, some may find that taking clen too close to bedtime prevents restful sleep, although this side effect tends to diminish with continued use. Most users prefer to administer clen at least 2X daily in equally divided doses, which I find to be superior for multiple reasons. Not only does it maintain more even blood levels of the drug, optimizing fat burning throughout the day, but it cuts down on the incidence of side effects, such as elevated heart rate and muscle tremors, which are commonplace in those who take their entire daily dose all at once. These side effects can be unpleasant, to say the least.

A typical daily dose for men falls in the range of 60-120 mcg/day, while 40-100 mcg/day is usually sufficient for women. Maximum doses should never be administered at the onset of treatment, but should be gradually titrated upwards as tolerance develops. A dosage of 20-40 mcg per day, in 2 equally divided doses, is a good starting point, with an additional 20 mcg added every 2-3 days until optimal dosing is reached. This drastically cuts down on the possibility of experiencing dangerous elevations in heart rate.

For those who desire to use clen for longer periods of time without experiencing down-regulation, they have the option of employing the 2nd generation antihistamine drug, ketotifin. Ketotifin works by preventing Beta 2 receptor down-regulation, allowing clen to exert its effects indefinitely. Although Clen can be used long-term under these circumstances, it is not advisable, as it increases the possibility of experiencing unwanted side effects.