This article comes from Chapter 27 of the book 'Healthy Weight Loss --> Healthy Life: The essential guide to long-term healthy weight loss for Australian adults'
Whilst pharmacies, health foods stores and even Multilevel Marketing (the newer version of pyramid schemes) ventures sell meal replacement shakes which work short term (but fail long-term) the other way in which they can makemoney selling weight loss is though weight loss pills, which are supposed to “boost your metabolism”, “burn extra fat” or even “block carbohydrate absorption” and "supress cravings"
You'd think no one would be silly enough to buy them, though if sales in the pharmacies I have worked in are any litmus, they would be very popular indeed. Occasionally, someone would ask me “do these things actually work?” I explained to them that they probably did very little if any good, were ridiculously expensive and would be almost completely worthless by comparison with healthy dietary modifications and an increase in exercise. When I then began trying to suggest how they could improve their diet and exercise habits, they would generally say “oh, okay”, before taking the weight loss pills to the counter and purchasing them regardless of what I just told them, which obviously wasn't what they wanted to hear.
The thing that appeals so much about these pills is that they are the quickest and easiest of all the “quick fixes”. You don't have to change your diet, you don't have exercise, and you don't have to undertake any effort other than having to pop a pill. How easy is that!? No wonder they are popular and no wonder people would prefer to try them instead of changing their diet or exercise (and ignoring my advice). But do they actually work?
Well, according to systematic reviews published by nutrition experts in the December 1999 issue of the Medical Journal of Australia, the November 2004 issue of the American Family Physician,  the April 2004 issue of the American Journal of Clinical Nutrition  as well as more recent studies, the answer is; no, not really. The only people who will be able to honestly say that they have lost a significant amount of weight whilst using them are people who were using them IN COMBINAION with other weight loss efforts such as diet and exercise, in which case it was their diet and exercise changes which caused them to lose weight; not the pills.
Most popular weight loss supplements do not contain 1 special ingredient; they usually consist of a variety of vitamins, minerals and herbs. There are many different brands of them available on the Australian market - 'Fat Blaster', 'Xantrax', 'Loss Accelerate' and 'Xantrax' (which sounds conveniently similar to a pharmacy only weight loss medicine which actually works) etc, though none of them are really that much different from each other, and all pretty much contain similar combinations of ingredients whilst charging outrageous prices. It wouldn't do much good to discuss each of the individual products separately, as they are all essentially the same thing. What I'll do instead is discus each of the most popular ingredients you'd be likely to find in them instead. Here goes:
Bitter orange contains alkaloid compounds similar to that found in Ephedra (a popular herbal weight loss supplement banned in most areas in the world as it was responsible for several deaths man many other dangerous side effects). Bitter orange has even been promoted as the “safer alternative to Ephedra” by some supplement companies pushing it as a weight loss supplement.
Bitter orange is supposed to help increase energy expenditure, though this claim does not appear to be supported by any scientific evidence. The only clinical trials which have tested the effect of Bitter orange on weight loss were small, contained methodological flaws and used it only as one of several combined ingredients; most often accompanied with caffeine and or ephedra (as described in a review study published in the February 2006 issue of Obesity Reviews ) Consequently, there is no good evidence to this date to suggest that it can help with weight loss. There have been no studies to evaluate whether it is safe to take long term.
Brindleberry (Garcinia cambogia/indica)
Brindelberry contains a compound called hydroxycitric acid which is supposed to act as an appetite suppressant as well as being able to inhibit the body from taking excess energy and turning it into fat. A handful of small, poorly designed studies which often used Brindelberry in combination with other constituents such as caffeine, reported small improvements in weight loss, however a larger study published in the 280th issue of the Journal of the American Medical Association found that it was not more effective than a placebo (a dummy pill).
Even if Brindelberry could somehow manage to stop the body from turning excess energy into body fat, wouldn't it make a lot more sense to simply avoid consuming excess energy? It would make more sense of course, but that would require a bit of effort. Popping a pill is much simpler isn't it?
Caffeine can help spare carbohydrate and instead utilize fat as an energy source. This is great if you are a long distance athlete, but whether or not it can actually help you burn a greater amount of total energy (which is, after all, the most important aspect to losing weight) or not is surely questionable. Most studies to examine the effect which caffeine has on weight loss have yielded conflicting results, and have usually included other ingredients such as ephedrine, which is banned in Australia due to it's potential for harm.
The only well designed study to test the effect of caffeine alone, was published in the April 1992 issue of the International Journal of Obesity Related Metabolic Disorders. The study found that after 24 weeks, subjects given 200mg of caffeine (equivalent to about 2-3 cups of coffee) had lost no more weight than those on the placebo.
Even if caffeine did help lose body fat and was not associated with unpleasant side effects (such as nervousness, irritability, heart burn, insomnia, raised blood pressure etc) wouldn't it be easier and less expensive to just have some tea or coffee? Some herbal weight loss supplements now contain Guarana, which is simply another source of caffeine; nothing to get excited about at all.
Capsaicin is the substance in chilies which gives them their hot, spicy taste. It is supposed to have a an appetite suppressing effect as well as a thermogenic effect, which is the ability to exhaust extra energy (Calories / kilojoules) as heat rather than store it as body fat.
One recent study published on April 25, in the International Journal of Obesity  found that people who took a supplement containing capsaicin as well as other ingredients (including caffeine and catechins) had lost less than 1kg more than those taking the placebo after 2 months. Hardly impressive, especially given that spicy foods may aggravate gastro esophageal reflux (heart burn) cause pain and even affect the quality of ones sleep.
There is no convincing evidence that capsaicin on it's own can have this effect, though even if it could; wouldn't it be better to simply add a bit of chili sauce to your dinner than forking out for expensive supplements?
Carnitine is a non-essential amino acid involved in the metabolism of fat. The hypothetical basis for its use in weight loss was that excess amounts could help promote the break down of fat. Quite simply, it doesn't.
Two studies have found that L-carnitine supplementation does not promote the break down of fat,[8,9] and to this date, no studies have examined whether it can promote weight loss. Evidently, this hasn't stopped supplement companies from promoting it as a weight loss treatment though.
Chitosan is a substance derived from crustacean shells which is purported to be able to “trap” or bind with fat in the intestines, therefore making it unavailable for absorption. Although animal studies have showed that it has this effect, a study published in the May 2003 issue of Obesity Research tested this theory by giving men approximately 133 g of fat to eat whilst taking 10 chitosan capsules per day. The results showed that only about 1g of fat was “trapped” and therefore not absorbed. Wow. Wouldn't it have made more sense to just eat a bit less fat each day?
A systematic review published in the Februrary 2005 issue of Obesity Reviews which examined the results of clinical trials evaluating the effect of chitosan on weight loss concluded that among high quality studies, “the effect of chitosan on body weight is minimal and unlikely to be of clinical significance.” 
Chromium is a trace mineral involved in maintaining proper insulin function. The intake of chromium among the Australian population is poorly understood, but it is hypothesized that if additional chromium could help to sensitize insulin more than usual, it may help to regulate blood sugar levels and therefore help minimize the occurrence of hunger associated with rapid blood sugar changes Due to its superior absorbability, chromium picolinate has been the supplement of choice for people trying to burn fat and or build muscle over the past decade or so, despite being found to be potentially clastogenic (damages DNA) in certain doses.
A meta-analysis of the 10 best designed clinical trials testing the effect of chromium picolinate on weight loss published in the April issue of the International Journal of Obesity Related Metabolic Disorders found that supplementing with large doses for about 3 months produced a very insignificant average loss of only 1.1kg. 
Basically, it's not worth it; especially given the potential for harm (many case reports have been noted of people who have experienced unfavourable side effects from large doses) 
Coleus Forskolin is another herbal medicine purported to “help support the metabolism of fat.” Only one, small clinical trial has ever been conducted to my knowledge. The results showed that the subjects given the supplement did not lose or gain any weight whilst those on the placebo did gain a little. The results were not clinically significant, and there have been no studies to test its long-term safety.
Fucus Visticulosis (Kelp)
Kelp is a type of seaweed which provides a source of iodine; a trace mineral needed for proper thyroid function. Whilst hypothyroidism (which promotes obesity) can result from inadequate iodine intake, there is certainly no evidence to suggest that consuming additional iodine can promote weight loss in people with normal intakes.  In most parts of Australia, this is not a problem.
Green tea is probably good for you, but there is little evidence that it will promote weight loss. A study published in the December 2005 issue of the British Journal of Nutrition found no difference in body weight among women given ghree tea or a placebo for 87 days. 
Phaseolus vulgaris (White Kidney Bean)
White Kidney Bean is supposed to inhibit amylase; the enzyme responsible for breaking down starch into glucose so that it can be absorbed in the small intestine. Only one small study involving 27 people has been conducted using white kidney bean, and was published in the March 2004 issue of Alternative Medicine Reviews. After 8 weeks, the subjects given the White Kidney Bean supplement has lost about 1.7kg's whilst those on the placebo had lost about 0.7 kg's; a difference which was not considered statistically significant.
Even if White Kidney Bean could help to stop excess starch being absorbed to contribute towards body fat storage, so long as you consume less energy (from any source) than what you use, you will lose weight. Wouldn't it therefore make more sense to simply avoid eating excessive amounts of starch in the first place? (Especially considering the cost of these supplements)
Psyllium Husk (Plantago ovata) and Guar Gum
Foods rich in soluble such as psyllium and guar gum tend to absorb water in the gut, slow gastric emptying and stabilize blood sugar. In significant portions, it is certainly plausible that a diet rich in these fibres may help control satiety. The tiny portions pound in weight loss pills however is so small and insignificant it is almost laughable. 
Furthermore, whilst a diet rich in soluble fibre may help with your long-term healthy weight loss goals, relying on psyllium and guar gum in the absence of other fibre-rich foods is unlikely to be of much assistance. Psyllium husk may help improve blood glucose and cholesterol levels; however a large clinical trial published in the September 1998 issue of the Journal of Diabetes and its Complications  found that it did not result in weight loss. Likewise, a meta analysis of clinical trials using guar gum for weight loss published in the June 2001 issue of The American Journal of Medicine  found that it was ineffective for weight loss.
There are various other ingredients which weight loss snake oil salesman will throw in every now and then, usually various vitamins as well as other herbs such as pyruvate, Gymnema Sylvestre, Ginger, Ginkgo biloba, soy extract, grape seed extract, lecithin, yohimbe and many others for which there is no good scientific evidence to suggest the ability to promote weight loss.[1-3]
The bottom line
Whilst these ingredients may not all necessarily be utterly useless, it is not reasonable to expect that either alone or combined, they are likely top help you lose any more than about 1 kg every few of months or so, and even then, would only do so on an energy restricted diet anyway. They are extremely expensive, even by comparison with other dietary supplements, and their benefit is barely significant by comparison with healthy eating and regular exercise. They appeal to people because they seem simple and effortless. Put simply, if you are really so lazy that you are going to rely on OTC weight loss pills instead of altering your diet and exercise habits, then I can assure you; nothing is going to help you. You are going to lose a lot of money and waste a lot of time and effort until you decide to get your priorities right and realize that there are no short cuts to long term healthy weight loss, so stop looking for them.
If you are adopting healthier eating and exercise habits, then you are going to be well on the way to reaching your weigh loss goals. Even if taking these pills could enhance your efforts by about 1%, why waste your money? If you are doing these things then you are going to lose the weight anyway. If you're in a hurry and think that this is going to help speed the process, then you're not thinking enough about long-term issues, in which case it isles likely that your new weigh loss attempts will be permanent, and more likely that they are just another yo-yo phase you're going to grow out of soon.
Sorry to be so harsh, but that's the way it is. Accept it and move on.
Watch out for fallacious arguments!
There are a few common argument made by promoters of herbal treatments, including (but not limited to) those supposed to help you lose weight. The most common fallacies these people use however is the argument that “natural” must be best, as well as the argument that they have been used for “thousands of years” by “wise, ancient cultures”
When I hear these people use this fallacious yet very effective marketing tool, I often think of a wonderfully natural herbal medicine used for thousands of years by the wise, ancient American Indians. European settlers soon began using it, and it became one of the most popular herbs used throughout the world. In the early part of last century however, some modern day scientists began to suspect that the herb may be harmful, but given that it was natural and had been used for thousands of years by wise ancient cultures, (the value of which is often regarded far more highly by new age followers than what science is) the initial claims were scoffed at by its proponents and especially the companies who sold it.
It took many decades of intense, scientific research to prove conclusively that it was harmful, after which time not only were the companies who sold it denying this evidence, but it still to this day has not been banned, despite being responsible for more deaths than any war has ever claimed. The wonderfully natural herb I am talking about of course is called tobacco, and unlike popular belief, the chemicals which cause heart disease, cancer and every of the disease in the book are not those added by the cigarette companies; they are those found naturally in the tobacco leaf.
Some of the most toxic substances known to man are perfectly natural. Anything can be harmful or harmless, depending on the dose in which you are exposed ot it. Whether it occurs naturally or not is completely arbitrary, yet it boggles the mind to come across so many people who prefer “natural” approaches because of their misbelieve that they must be safer; they aren't. So why then do they not carry the horrible warnings on their labels like many prescription drugs do?
There are two reasons for this. The first is that in this country, so long as they are considered a “listed” therapeutic good (as apposed to a registered one) they get to enjoy a different category or regulation. They do not need to be studied to be able to be sold, so long as they have been used for “thousands of years by wise ancient cultures”, even is these wise ancient cultures certainly never used them in the highly concentrated mega-doses available in the”health food “s store, nor did they use them as a daily supplement as we seem to think is okay to do. Some of the substances sold as listed therapeutic goods have not even been studied in animals for goodness sake. Even if they were harmful, unless they were intensely studied, no one would know, especially if the harm came from chronic exposure (similar to that which occurs from years of smoking tobacco, which incidentally, took many years to prove it's harm).
The other reason is that as a listed therapeutic good (which supplements are) they are not required to list warnings on the labels with the exception of selenium and retinol. Depending on the dose, there is evidence to suggest that many of the most popular vitamins, minerals and herbs may be potentially toxic if taken long term. Because supplement companies are not forced to list warnings on the labels, they don't; it would only turn away potential customers under the impression that they must be safe.
If you're really intent on using dietary supplements or herbal medicines, see an appropriately qualified health care professional; preferably one who doesn't have a financial interest in selling you things you don't need.
 Egger G, Cameron-Smith D, Stanton R. The effectiveness of popular, non-prescription weight
loss supplements .Med J Aust. 1999 Dec 6-20;171(11-12):604-8 <full text>
 Saper RB, Eisenberg DM, Phillips RS. Common dietary supplements for weight loss. Am Fam Physician. 2004 Nov 1;70(9):1731-8. <full text>
 Pittler MH, Ernst E. Dietary supplements for body-weight reduction: a systematic review. Am J Clin Nutr. 2004 Apr;79(4):529-36. <full text>
 Haaz S, Fontaine KR, Cutter G, Limdi N, Perumean-Chaney S, Allison DB. Citrus aurantium and synephrine alkaloids in the treatment of overweight and obesity: an update. Obes Rev. 2006 Feb;7(1):79-88. <full text>
 Heymsfield SB, Allison DB, Vasselli JR, et al. Garcinia cambogia (hydroxycitric acid) as a potential antiobesity agent: a randomised controlled trial. JAMA 1998; 280: 1596-1600. <abstract>
 Astrup A, Breum L, Toubro S, et al. The effect and safety of an ephedrine/caffeine compound compared with ephedrine, caffeine and placebo in obese subjects on an energy restricted diet. A double blind trial. Int J Obes 1992; 16: 269-277.
 Belza, A.Frandsen, E., and Kondrup, K. Body fat loss achieved by stimulation of thermogenesis by a combination of bioactive food ingredients: a placebo-controlled, double-blind 8-week intervention in obese subjects Int J Ob Apr 2006; 30 (4): 579 – 727 <abstract>
 Vukovich MD, Costill DL, Fink WJ. Carnitine supplementation: effect on muscle carnitine and glycogen content during exercise. Med Sci Sports Exerc 1994; 26: 1122-1129. <abstract>
 Sulkers EJ, Lafeber HN, Degenhart HJ, et al. Effects of high carnitine supplementation on substrate utilization in low-birth-weight infants receiving total parenteral nutrition. Am J Clin Nutr 1990; 52: 889-894. <full text>
 Gades MD, Stern JS. Chitosan supplementation and fecal fat excretion in men. Obes Res. 2003 May;11(5):683-8. <full text>
 Mhurchu CN, Dunshea-Mooij C, Bennett D, Rodgers A. Effect of chitosan on weight loss in overweight and obese individuals: a systematic review of randomized controlled trials. Obes Rev. 2005 Feb;6(1):35-42. <abstract>
 Pittler MH, Stevinson C, Ernst E. Chromium picolinate for reducing body weight: metaanalysis of randHomized trials. Int J Obes Relat Metab Disord. 2003 Apr;27(4):522-9. <abstract>
 Jeejeebhoy KN. The role of chromium in nutrition and therapeutics and as a potential toxin. Nutr Rev. 1999 Nov;57(11):329-35 <abstract>
 Henderson, et al., Effects of Coleus Forskohlii Supplementation on Body Composition and Hematological Profiles in Mildly Overweight Women. Journal of the International Society of Sports Nutrition. 2(2): 54-62, 2005. <full text>
 Diepvens K, Kovacs EM, Nijs IM, Vogels N, Westerterp-Plantenga MS. Effect of green tea on resting energy expenditure and substrate oxidation during weight loss in overweight females. Br J Nutr. 2005 Dec;94(6):1026-34. <abstract>
 Udani J, Hardy M, Madsen DC. Blocking carbohydrate absorption and weight loss: a clinical trial using Phase 2 brand proprietary fractionated white bean extract. Altern Med Rev. 2004 Mar;9(1):63-9. <abstract>
 Rodriguez-Moran M, Guerrero-Romero F, Lazcano-Burciaga G. Lipid- and glucose-lowering efficacy of Plantago Psyllium in type II diabetes. J Diabetes Complications 1998;12:273-8.
 Pittler MH, Ernst E. Guar gum for body weight reduction: meta-analysis of randomized trials. Am J Med 2001;110:724-30. <abstract>