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Diet Pills

: A Research Summary (Jan. 2017)

By: Kelly Burgess on January 23, 2017

Editor's note:
Diet pill sellers want you to think you can just take a pill and your fat will melt away, but, in fact, most of those claims run the gamut from misleading to outright lies. Only five weight loss drugs have actually been approved by the FDA, and even those show only modest results. Before you enrich a TV charlatan, learn what science says about diet pills.

  • Some studies show mild weight loss
  • Generally safe when taken at low doses for short periods of time
  • Potential for serious side effects
  • Some are dangerous
  • Can make you feel anxious and jittery
  • Insufficient reliable evidence to recommend their use

Thermogenics don't necessarily burn fat as their marketing name -- fat burners -- suggests. What little help they do offer is a result of stimulating the sympathetic nervous system to increase the amount of energy your body expends while at rest. The sympathetic nervous system is the part of our nervous system responsible for the "fight or flight" response, which increases heart rate and blood pressure. If thermogenics are taken in large amounts or for extended periods, these heart rate and blood-pressure increases can have serious side effects, including strokes, irregular heart rhythms and even heart attacks. Science doesn't show that these risks are worth the nominal weight loss that these products may produce, and the risks increase when two or more ingredients are combined. This brings up two points of caution when using thermogenics:

1)     Watch out for products that contain multiple ingredients, as those formulations carry increased risk of serious side effects.

2)     Be aware of how much of one active ingredient you're taking -- such as caffeine, which is found in guarana and green tea -- so you can avoid an overdose. 

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  • Minimal side effects
  • Could keep you from having to completely eliminate carbohydrates
  • Insufficient evidence to recommend
  • Can cause some gastrointestinal upset

Fat blockers work as their name suggests by blocking the absorption of fat in the intestines. Chitosan is one of the more well-known ingredients that perform this function, and it is made from the shells of crustaceans. The lesser known alginate is found in brown algae. Neither has been proven as a weight loss aid. Further research is needed, but if the data continues to come in the way they have in the majority of studies, neither chitosan nor alginate will be recommended for use in weight loss. Also, because they interfere with fat absorption, they can potentially interfere with the absorption of medications. Lastly, chitosan should be avoided if you have an allergy to shellfish.

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  • Minimal side effects
  • Insufficient weight loss proof
  • Not good for those allergic to shellfish
  • May interfere with medications

Fat blockers work as their name suggests by blocking the absorption of fat in the intestines. Chitosan is one of the more well-known ingredients that perform this function, and it is made from the shells of crustaceans. The lesser known alginate is found in brown algae. Neither has been proven as a weight loss aid. Further research is needed, but if the data continues to come in the way they have in the majority of studies, neither chitosan nor alginate will be recommended for use in weight loss. Also, because they interfere with fat absorption, they can potentially interfere with the absorption of medications. Lastly, chitosan should be avoided if you have an allergy to shellfish.

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  • Some have been used for centuries
  • Insufficient evidence to recommend their use
  • Potential serious side effects, such as esophageal or intestinal obstructions and liver damage.

Appetite suppressants curb your hunger; if you eat less, you'll lose weight. However, of the ingredients that we studied, there wasn't sufficient evidence to recommend their use. The most interesting are caralluma and hoodia, which have been used for centuries by indigenous tribesmen to suppress appetite, but further research is needed to see if they can be proven clinically effective. Some appetite suppressants have the potential for serious side effects: Hydroxycitric acid (HCA) has the potential to cause liver damage; glucomannan and guar gum if taken in large amounts and without water can potentially cause esophageal and intestinal obstructions; and the use of 5-hydroxytryptophan (5-HTP) has been associated with a condition that affects blood cells and muscles called eosinophilia myalgia syndrome (EMS). It is recommended that use of 5-HTP be avoided until further research is completed.

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  • Used under doctor supervision
  • Overseen by Food and Drug Administration
  • Proven modest weight loss
  • Various side effects
  • Not many choices
  • Can be expensive

Prescription drugs could be the gold standard (if there was one) of the diet pill industry. Regulated by the Food and Drug Administration (FDA), companies are required to prove that their weight loss products either produce weight loss that is greater than 5 percent (compared to people taking a placebo) or help more than 35 percent of the population achieve greater than 5 percent weight loss; in addition, lipid, glycaemia and blood pressure profiles must improve. These requirements have made the options available limited. The few products that are available are associated with 3 percent to 5 percent loss of bodyweight over six months to one year and improved cardiovascular benchmarks, such as blood pressure.

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Is it worth living in a stressful state to lose weight?

Just the name "fat burner" as a dietary supplement category elicits the image of your body becoming an inferno to melt away the inches of fat surrounding your belly and thighs, but that isn't the case. Many of these supplements, also known as thermogenics, are meant to increase your metabolism, or energy expenditure, by stimulating your sympathetic nervous system. Best known for controlling our fight or flight reaction, the sympathetic nervous system prepares us to react to a stressful situation by preparing the body for action -- increasing heart rate, blood pressure, blood flow to the brain, body heat production (aka thermogenesis), basal metabolic rate (the number of calories our body burns at rest) and more. Furthermore, the sympathetic nervous system regulates the body's process of breaking down fat (fat oxidation) for energy, which may play an important role in the regulation of total body fat, as well as working every hour of every day to keep the body at homeostasis, or its usual equilibrium.

Thermogenics stimulate the sympathetic nervous system to burn more energy while at rest and are known as stimulants. One of the most well-known is caffeine, and it is the active ingredient in many of the diet supplements listed in this category; for instance, green tea and guarana (Paullinia cupana). Another commonly known thermogenic is ma huang, also known as Ephedra sinica. It was banned in 2005 by the Food and Drug Administration (FDA) because of serious side effects, or unintended consequences of its use. Ephedra has been replaced by synephrine -- a compound similar to ephedra -- that serves as the main active substance in bitter orange (Citrus aurantium). Other thermogenic supplements include capsaicin, Cirsium oligophyllum, and yohimbine (Pausinystalia yohimbe). Some of these thermogenic diet pills have been shown to produce a small amount of weight loss, but the question you should be asking is: Are they worth the risk?

For example, the combination of ephedra and caffeine is known to cause very modest weight loss, but it's also known to cause insomnia, nervousness, tremors, increased blood pressure, irregular heart rates and, more seriously, strokes, heart attacks and death. According to American Family Physician, between 1997 and 1999 alone, there were 87 reports made to MedWatch, an FDA program, about ephedra, of which 23 occurred in patients with no previous health problems who were taking the recommended dose. Ten of the 87 events reported while using ephedra led to death, and 13 led to permanent disability. The use of ephedra helped patients lose an additional 2 pounds per month.

Ephedra has been banned, but questions remain about whether its cousin, synephrine is any safer. According to the National Institutes of Health, some of the reported side effects of synephrine include chest pain, anxiety, and increased blood pressure and heart rate. In addition, many diet pills have multiple ingredients, and you may end up taking more than one thermogenic, which can have serious consequences. For more on the efficacy and risks of each thermogenic, please see below.


What is it? Caffeine is a chemical compound found in multiple plants, namely those used to make coffee, chocolate and tea. According to experts, about 90 percent of Americans consume caffeine every day via sodas, energy drinks, some headache medications and diet pills. Caffeine is the main active ingredient in guarana (Paullinia cupana) and one of the active ingredients in green tea (Camellia sinensis). According to the Natural Medicines Comprehensive Database (NMCD), caffeine is found in 2,169 products on the market.

Does it work? Caffeine has most often been studied in combination with other thermogenics such as ma huang, better known as Ephedra sinica (which has been banned by the FDA), guarana and yerba maté. Although it might be effective in producing weight loss in the short term, according to clinical studies, more research is needed. That's largely because the most effective combination -- ephedra and caffeine -- is no longer available. Caffeine is also known to be an efficient diuretic, and the loss of water weight can look like weight loss when viewed in the short term. For each milligram of caffeine consumed, approximately 1.17 milliliters (mL) of water is excreted through the urine.

Is caffeine safe? Caffeine is safe when used in small amounts; however when taken in large doses it can cause tremors, delirium, agitation, chest pain, an irregular heart rhythm and an increase in heart rate and breaths per minute. At doses of 150 to 200 milligrams (mg) per kilogram (kg) of body weight, about 10,000 to 14,000 mg for an average-weight adult, caffeine can be fatal. (By comparison, a large brewed coffee from Starbucks contains 320 mg of caffeine.) Multiple ingredients in diet pills can contain caffeine, including green tea extract, guarana and yerba mate, and you can also get caffeine from other sources, such as coffee (as noted above), soft drinks and energy drinks. Caffeine can interact with various other supplements and medications, especially other stimulants. Speak to your healthcare provider before starting a supplement that contains caffeine.

Green tea (camellia sinensis)

What is it? Green tea is made from Camellia sinensis plant leaves. The main active ingredients in green tea are believed to be the antioxidant epigallocatechin gallate (EGCG) and caffeine. Most often consumed as a beverage, green tea is also available in pill form as an extract. According to the NMCD, there are 3,835 products on the market that contain green tea. The amounts of EGCG and caffeine can vary in each product because growing conditions, leaf age and storage can have an effect. Some extracts will state what percentage of EGCG and/or caffeine is present, but typically a cup of green tea will contain between 80 and 100 milligrams (mg) of EGCG and between 10 and 80 mg of caffeine.

Does it work? It's unclear, but green tea extract may help weight loss efforts in certain circumstances. Some preliminary studies (human and animal) showed that green tea increased thermogenesis and the basal metabolic rate (BMR); clinical studies in humans, however, have produced conflicting results. Some studies have shown no weight loss when using green tea compared to a placebo, but others have shown a moderate reduction in weight, body mass index (BMI) and body fat when taking green tea in conjunction with exercise -- even if the tea was decaffeinated. In addition, it appears that green tea when taken in conjunction with caffeine can reduce weight, BMI and waist circumference more effectively than when caffeine is taken alone. However, the studies that have been conducted are small, and more research needs to be done before a definitive answer can be given as to green tea's efficacy.

Is green tea safe? The National Institutes of Health rate green tea (the drink) as likely safe, but green tea extract as only possibly safe. Studies show that there are minimal adverse events of consuming green tea in moderate amounts -- less than six cups of tea per day for less than six months. Reported side effects are mild and include vomiting, abdominal bloating, gas and diarrhea. However because green tea contains caffeine, taking high doses for extended periods can cause unpleasant symptoms, such as insomnia, agitation, tremors, and restlessness. With higher doses, one possibly could experience irregular heart rhythms and heart failure. Therefore, it is important to keep track of the other sources of caffeine in your diet to prevent an overdose -- approximately 10 to 14 grams (g) for an average adult.

More concerning to experts is the link between green tea extract and liver damage or liver failure. In most of these cases, the liver function returned to normal when the product was no longer used, but there have been instances in which a liver transplant was required, as well as several reported deaths. Green tea extract can also interact with various supplements and medications, especially those that are also stimulants. Before taking green tea as a supplement, speak with your health care provider.

Guarana (Paullinia cupana)

What is it? Guarana is made from the seeds of the Brazilian plant Paullinia cupana, also known as Brazilian cocoa. It contains 3 to 5 percent caffeine, and this is the main ingredient believed to confer the weight loss benefits. According to the NMCD, there are 1,101 products on the market that contain guarana. Guarana is also a popular ingredient in soft drinks, especially in South America.

Does it work? Probably not -- at least on its own. Preliminary studies of diet pills that contain guarana along with other ingredients suggest that it may be effective at producing modest weight loss, about 6 pounds in two months, but it is unclear whether these results are due to guarana or one of the other ingredients. More research is needed.

Is guarana safe? Mostly. Guarana's high concentration of caffeine is the root of any side effects. Like caffeine, mild consumption causes minimal side effects, but larger doses can cause insomnia, nervousness, elevated heart rate, tremors, irregular heart rhythms and chest pain. At doses of 150 to 200 milligrams (mg) per kilogram (kg) of body weight, about 10 to 14 grams (g) for an average adult, caffeine can be fatal. Be aware of what other dietary supplements, foods and drinks you are consuming that contain caffeine. Guarana can interact with any other herbal supplement or medication that is considered a stimulant as well as other medications. Speak to your healthcare provider first before starting a supplement that contains guarana.


What is it? Synephrine mimics the effects of the sympathetic nervous system, making it a sympathomimetic agent. It is chemically similar to phenylephrine, a medication used to decrease congestion and increase blood pressure. Synephrine is found in several products, including the ingredient bitter orange (Citrus aurantium).

Does it work? It may, but evidence as to whether synephrine is effective at producing weight loss is conflicting. Synephrine has most often been studied in conjunction with bitter orange and caffeine. While one study suggests that it might help reduce weight when combined with diet and exercise, another study shows no significant weight loss with its use. More research is needed.

Is synephrine safe? Possibly not, as side effects include high blood pressure, increased heart rate, irregular heart rhythm, heart attack, stroke and sudden death. Because of its similarities to ephedra, many experts and medical professionals say that synephrine should be banned as well. Synephrine can have significant interactions with other herbal supplements and medications that have stimulant effects. If you do choose to use a product with synephrine, definitely speak to your healthcare provider beforehand. As an aside, the National Collegiate Athletic Association (NCAA) has banned the use of this substance.

Bitter orange (citrus aurantium)

What is it? The bitter orange (Citrus aurantium) is also known as the Seville orange or the sour orange. The fruit and peel are used for weight loss, and their main active ingredient is synephrine, a chemical that stimulates the sympathetic nervous system. For more information, please see our discussion of synephrine just above this section.

Does it work? It could be effective, but only time will tell. A few studies show weight loss of between 4 and 6 pounds over several months, and some studies have shown that bitter orange has no effect on weight loss. Additional studies have been done on diet pills with multiple ingredients including bitter orange. While these studies have had mixed results, it is unclear whether the positive results are due to bitter orange or to any of the other ingredients. More research is required.

Is bitter orange safe? Probably not. The main active ingredient in bitter orange is synephrine, which as we note above, has enough worrisome side effects that many health professionals would like to see it banned. In addition, bitter orange often has other stimulant ingredients as well. As with synephrine, the NCAA has also banned the use of bitter orange by its athletes.


What is it? Capsaicin, also known as capsicum, is the chemical responsible for making peppers spicy. Its pungent taste can produce a burning sensation on the tongue. It stimulates the sympathetic nervous system, thereby increasing thermogenesis, or heat production, according to animal and human research. According to the NMCD, there are 1,812 products on the market containing capsaicin.

Does it work? Possibly. Clinical studies in humans have shown an increase in energy expenditure following the intake of capsaicin after a meal. In addition, some studies have shown a reduction in body fat and weight following the use of capsaicin. However, researchers discovered that patients had difficulty taking the full dosage because of its pungency, making it ineffective for long-term use. Further research is needed.

Is capsaicin safe? Yes, to a point. When used at doses typically found in food, there is minimal safety concern; however long-term use, especially of high doses (more than 120 mg a day), may be linked to kidney, liver and gastrointestinal damage. Side effects that can occur at lower doses include abdominal discomfort, gas, nausea, heartburn, diarrhea, and sweating. There are few interactions with medications, but, theoretically, capsaicin can increase the effects of blood thinners such as Coumadin (warfarin), Lovenox (enoxaparin) and Plavix (clopidogrel). Before using capsaicin at higher doses, or if you're on any other medications, speak to your healthcare provider.

Cirsium oligophyllum

What is it? Cirsium oligophyllum is a type of thistle commonly found in eastern Asia and central Japan. Research is relatively minimal, but it is believed that cirsium activates the sympathetic nervous system like other thermogenics, thereby promoting the breakdown of fat cells.

Does it work? The jury is just starting to deliberate on this ingredient. Only one scientific study could be located, and this study was conducted on rats. While the study showed promising results for the use of cirsium in weight loss and reducing body fat, it is difficult to say that these same results can be expected in humans. Further research is necessary.

Is cirsium oligophyllum safe? It is unknown at this time. Such minimal research has been done into cirsium that there is not enough data on adverse events or side effects of its use.

Yohimbine (pausinystalia yohimbe)

What is it? Yohimbine (pausinystalia yohimbe) is a stimulant derived from the ground bark of a tall evergreen native to Central Africa. It is most commonly used as an aphrodisiac. There are 669 products containing yohimbe on the market, according to the NMCD, though many of them are in the sexual aid category.

Does it work? There isn't enough evidence to say yes or no, because only three randomized clinical trials have been conducted. Their results are conflicting, and more research is required to determine whether yohimbine is a viable weight loss aid.

Is yohimbine safe? It's questionable. Yohimbine has been associated with some serious adverse effects, including agitation, irregular heart rhythms, heart attacks and seizure. Yohimbine can interact with other herbal and prescription stimulants and has been known to have an additive effect to monoamine oxidase inhibitors (MAOIs). Do not take yohimbine unless you are under a doctor's supervision.

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