What They Are
Appetite suppressants are products that use various combinations of drugs to promote weight loss by manipulating hormonal and chemical processes in the body that control hunger pangs and the sense of feeling full or satiated. The technical name for these drugs is "anorectic" from the Greek anorektos, meaning "without appetite."
Feeling Less Hungry
Anorectics that help reduce hunger pangs prior to the onset of a meal are referred to as noradrenergic drugs because of the two hormones they cause the adrenal glands to release: epinephrine and its close relative, norepinephrine (also known as noradrenalin). These are both known as "fight-or-flight" hormones because of the manner in which they stimulate the central nervous system and increase the heart rate and blood pressure. These hormones also interrupt the brain's signal to the body that it is time to feel hungry. Most of the drugs used to stimulate the production of these hormones are closely related to amphetamines and can sometimes be recognized by the appearance of "phen" or "phed" somewhere in their name. Examples are phentermine (used in Fastin, Adipex, and Lonamin) or norpseudoephedrine.
Feeling Full Sooner
Anorectics that help to quicken the feeling of fullness or satiation once a meal has begun frequently make use of serotonin re-uptake inhibitors, which are widely used in anti-depressants because of how they increase the general sense of well-being. Examples of anorectics that include serotonin re-uptake inhibitors are sibutramine (used in Meridia and Reductil) and rimonabant (used in Acomplia). When the serotonin re-uptake process is inhibited or slowed, it can reduce the need to overeat in pursuit of a sense of physical (and emotional) satisfaction.
What Problems They Pose
The two main drawbacks with using anorectic drugs is that their effectiveness can decrease as weight is lost, and they can have many undesirable side effects, ranging from the jitters all the way up to hypertension and heart-valve damage. There is much that we do not fully understand about the physiological effects of anorectic drugs, and so much more that is difficult to measure in terms of the emotional components to eating that can have a tremendous impact on the effectiveness of appetite suppressants. The bottom line is that if you are considering taking an appetite suppressant, it is important not only to involve your doctor, but to do plenty of research on your own so that you fully understand what potential benefits as well as risks you are exposing yourself to.
Some weight loss products on the market also employ the use of fat absorption blockers or "natural" ingredients such as green tea extract, but neither of these are "appetite suppressants" in the strict clinical sense. Do your homework before trying these products as well, because weight-loss products claiming to be "natural" are not always as natural as they might seem at first glance, and products that block fat absorption can interfere with other aspects of food digestion and nutrient absorption.