If you've ever talked to a bodybuilder about what he takes for fat loss, you've probably heard the word clenbuterol before. It's one of the most misunderstood tools in the bodybuilder's toolbox, so hopefully this article will clear up any false information that's floating in the ether known as the world wide web.
Clenbuterol is not a steroid, but often associated with anabolic steroids due to the demographic of users. Clenbuterol is an asthma medication that mostly was marketed in Europe.
Related - Ultimate Guide to SARMs
Clenbuterol, also called "clen," is a part of a group of medications known as sympathomimetics, which are drugs that affect the sympathetic nervous system in a variety of ways, but mostly by interacting with your adrenoreceptors.
There are alpha and beta adrenoreceptors, where alpha receptors are normally associated with stimulation of effector cells and constriction of blood vessels and beta receptors are involved in the relaxation of effector cells and dilation of blood vessels.  There are two subtypes of alpha receptors (β1 and β2) and there are 3 subtypes of beta receptors: β1, β2, and β3.
Clenbuterol is very specific to activating your beta-2 receptor which elicits the relaxation of smooth muscle tissue in the lungs (known as bronchodilation). One side effect of beta-2 activation, however is increased heart rate and heart muscle contraction (known as tachycardia).
Clenbuterol's mechanism of action for fat loss is very similar to other beta-2 receptor agonists, like ephedrine. Once clen activates the beta-2 receptor, it signals the body to increase cellular levels of cyclic AMP (cAMP). In turn, cAMP induces the activity of protein kinase A, which then stimulates the fat burning enzyme hormone sensitive lipase (HSL).
HSL initiates the process of breaking down fat cells for energy known as lipolysis.  There is even data suggesting that clen can have downstream activation on PPARa and UCP1, both are proteins involved in energy regulation and fatty acid oxidation in cells. 
Looking at the structure of clenbuterol, you can see it's very similar to another extremely popular fat loss agent, ephedrine. Ephedrine is also primarily a beta-2 adrenergic receptor agonist and burns fat via the same mechanism.
Weight loss studies with subjects using ephedrine and caffeine show an increase in basal metabolic rate of around 8%. A similar increase in energy expenditure can be expected when using clenbuterol, which is obviously dose-dependent. 
That means without changing diet or exercise, taking clenbuterol can burn an extra 200 calories a day for someone who normally burns 2500 calories a day. This may not seem like a lot, but add in a strict diet, caloric deficit and exercise, the results on body composition can be quite dramatic.
Can Clen Add Muscle?
There has been a lot of "bro science" talk about how clenbuterol can increase muscle mass. I think the origin of this is from animal studies showing rats gaining muscle tissue when administered clenbuterol.
Here's what you need to ask though: what's the mechanism of action for stimulating muscle protein synthesis and what is the dose of clenbuterol used? The answer to the first question is multifaceted, but there is, in fact a mechanism shown to increase muscle mass via clenbuterol supplementation.
Clenbuterol seems to directly activate the mTOR dependent pathway to increase muscle protein synthesis. It also can decrease the rate at which muscle tissue is broken down. In fact, this effect was noticed even when subjects were in a 50% caloric deficit for long periods of time. 
The answer to the second question is a little misleading. In a study done on rats supplementing with 2mg/kg of clenbuterol, researchers noted an increase in muscle mass in the rats. However, if you convert 2mg/kg to the human equivalent dose, you get 10.54mg (milligrams, not micrograms) of clenbuterol which might induce a heart attack in humans. 
A study in humans though, told a much different story. 19 subjects taking 80mcg of clenbuterol a day showed a statistically significant increase in lean body mass, while losing body fat to maintain the same overall body weight. 
Side Effects of Clenbuterol
Clenbuterol does not come without side effects and can be dangerous if not used correctly. Studies in animals have shown side effects ranging from tachycardia and increased heart rate, to myotoxic effects causing cellular death of heart muscle.
Chronic use of clenbuterol can even adversely affect cardiac function.  Muscle cramping is also a common side effect of clenbuterol use, as clen depletes serum and muscle tissue levels of taurine. Supplementing with 2.5-5g of taurine a day can help mitigate cramping and is recommended. 
How to Take Clen
Clenbuterol has a half-life of around 36 hours, which is an extremely long time compared to ephedrine which has a half-life of around 4 hours. With that being said, clenbuterol only needs to be taken once a day as a fat loss agent.
Multiple dosing will only exacerbate the side effects of elevated heart rate, tachycardia, profuse sweating, ect. Real clenbuterol tablets come in a strength of 20mcg (micrograms, 1000mcg = 1mg) and it is usually advised to start off with one 20mcg tab a day and slowly increase the dose until the desired effect is reached.
Maximum daily dose should not exceed 120mcg. There is also some confusion on how many consecutive days one can take clenbuterol before it ceases to work for beta-2 activation.
Clenbuterol seems to quickly down-regulate expression of your beta-2 receptors at around 18 days of consecutive use, so a "2 days on, 2 days off" or "2 weeks on, 2 weeks off" protocol was introduced. This may help as clenbuterol has a very long half-life. 
However, a more chemistry approach would be to take the antihistamine drug ketotifen. In a study that concurrently gave asthma patients clenbuterol with and without ketotifen, it was noted that the ketotifen group the beta-2 receptor function was upregulated. 
Therefore, if you're going to take clenbuterol for more than 2 weeks, I would suggest adding a nightly dose of 1-2mg of ketotifen to keep beta-2 receptor activation maximized.
RUMOR ALERT: I don't know how this started but someone decided to say that since ketotifen is an antihistamine and it can help prolong clenbuterol's effective period, then Benadryl (diphenhydramine) should do the same thing, right? WRONG!
There are tons of similar drugs in the same classes that do not elicit the same effects, like albuterol and clenbuterol. Please do not take Benadryl and think it's helping with your clen cycle because it's not.
Notice I said "real" clenbuterol. There are a lot of research chemical companies selling "clenbuterol" that is really a mixture of albuterol and some type of stimulant. I would only buy clenbuterol in a tablet form or the Ventipulmin pump, which comes in a 355mL pump with a single dose of 25mcg.
If you are taking clenbuterol, I would also advise avoiding or severely cutting back on other stimulants like caffeine, and never stack with ephedrine! Clenbuterol is a very strong beta-2 agonist and can be uncomfortable at times to use.
I can remember sitting in class with my hands shaking and sweating profusely when only taking 40mcg of clen. Do not underestimate its strength and as always be safe!