"Clenbuterol is a sympathomimetic that works on the sympathomimetic nervous system. There are several receptors in the body a sympathomimetic can act on. In the case of Clenbuterol, the beta-2 receptor is the area of interest and action. Clen, as it’s commonly known, actively stimulates the beta-2 receptor. Through such stimulation, this actively reverses airway obstructions and provides improved breathing for those who need it. This same stimulation can also be used to enhance the metabolic rate of the individual. Clenbuterol does not actively burn fat by attacking fat cells, but rather stimulates the metabolism by increasing the body’s temperature. This occurs due to the beta-2 stimulation stimulating the mitochondria of the cells to produce and release more heat. In turn, this heats up the body’s temperature (slightly), enhances the metabolism and causes the individual to burn body fat at a greater rate. In reality, the functions and traits of Clenbuterol are very simple and very straightforward.
There are several possible side effects to Clenbuterol use, and the primary will surround its stimulating nature. No surprise, after all, Clen is a stimulant. The side effects of Clenbuterol can be very strong and often very annoying to say the least, and they will generally affect most people in one way or another. The most common side effects surround a jittery or wired feeling, shaky hands and increased sweating. Most all who use Clen will experience such effects to a degree, but they will commonly be very pronounced during the early stages of use. As the individual becomes accustomed to the stimulant, such related effects should begin to subside, but they will be so strong in some people that some will not be able to use it.
When using Clenbuterol, many often inaccurately assume that when the stimulating effects of Clen began to fade that the thermogenic effects are no longer working. Most will find that the stimulating effects will greatly subside after a week or so of use, but assuming the fat burning properties have vanished due to this lessened stimulation is highly inaccurate. The same dose of Clenbuterol Hydrochloride can actually keep the metabolism revved for as much as five weeks. Granted, by the fifth week mark it will be minimal as the body does most certainly adapt. Due to the body’s ability to adapt, necessarily adjustments must be made to use.
The aforementioned side effects of Clenbuterol are the most common, but there are other possible effects of note. As with many stimulating substances, some will experience headaches as well as possible nausea. One of the more bothersome possible side effects of Clenbuterol Hydrochloride will be muscle cramps. Cramps are not exceedingly common, but they will affect many people. Athletes are often at the greatest risk due to the hard and often intense training they are already undertaking. Staying well hydrated is often enough to avoid or remedy this problem, but supplementing with taurine can also help. Clen has been shown to deplete taurine, so supplementation may be necessary for some. Some may also have issues with insomnia; in fact, some may find sleep becomes extremely difficult. Again, this is very common with many stimulating substances, but with Clen it can, for some, be impossible to avoid. Clen has an active half-life that stretches to the 34 hour mark, and that can make sleep impossible for some people. Many will, however, find if they take their Clen first thing in the morning they will not have any issues with sleep, but due to the long half-life some will find insomnia occurs regardless of when they take it.
The use of Clenbuterol Hydrochloride also carries with it possible side effects that can be severe; in fact, dangerous would be a more accurate description. Such effects are most commonly associated with abuse through high doses and far beyond recommended extended periods of use. The severe side effects of Clenbuterol include high blood pressure, irregular heartbeat, trembling and even panic. Some studies have also shown that Clenbuterol abuse can also lead to cardiac hypertrophy, which could potentially lead to death. It is very possible to use this compound without such effects, but as with so many things in life it will require responsible use and a thorough understanding of Clen."
Source: various online articles, not my own writing.
BEFORE you start taking such strong supplements to aid in fat loss, PLEASE make sure your diet is down perfect, or one, you will waste the true effects of the drug, and two, you will do more harm than good in the long run. I won't take the same position as some people that strongly advocate against "steroid" like drugs (clen is NOT a steroid) but since you're asking for the side effects of a drug that is WELL documented on the internet and online journals, this gives me the impression that you're a beginner. As a result, I STRONGLY advise you to maximize your fat loss through proper caloric deficit diet and high intensity exercise FIRST, once you do that, then clen can help you lose the last bit of fat very easily. Think of it this way, if you want to get better at math, what is the point of learning calculus, if you don't even know simple arithmetic ? You should have at least a few years experience dieting properly and you should already be relatively lean before you start. You DO NOT need this drug to lose fat if you are already above, let's say 20-30%, that part can easily be lost through natural diet and exercise.
Please note also that, if you don't have your diet down, you are likely to lose way more muscle than fat and therefore end up even unhealthier than when you first began the drug.