Bodybuilding is a field of endeavor filled with woo. Many bodybuilders don't understand science, and consider anecdotal evidence and personal recommendation good enough to try something out and then advocate it to others if they gained while on it.
Bodybuilders understand "no pain, no gain" and have no problem working hard. The trouble is how to work smart as well.
Dietary supplements are a lucrative industry and include substances like creatine, amino acids and protein supplements, plant sterols (sold as legal purported alternatives to anabolic steroids, which are illegal), packets of high-dosage vitamin pills claimed to have "anabolic" or "metabolic" abilities, anti-oxidants, and (before their inclusion in bodybuilding supplements was banned), mixes containing ephedrine (often as an "ECA stack").
As a general principle, anything sold OTC at the mall promising to deliver results in line with or exceeding drugs is suspect. Besides speed (see ECA above), most lack proven or even plausible mechanism. Some are happily pitching megadose broscience while others are hazardous levels of a stimulant and others would be royally hazardous if their mechanism did work (such as telomerase activators or edible "low T" treatments). These categories will overlap. Do not expect much help from the hourly employee at GNC whose job is to sell you stuff.
Another problem with bodybuilding supplements is that they are often adulterated with anabolic steroids or aromatase inhibitors, which are not listed on the labels because they are illegal without a prescription. These random, uncontrolled contaminants may cause temporary changes in a metric or you to fail a drug test, they are drugs. Taking random quantities of random stuff isn't exactly something to train off of though. As below, actual use of dope for athletic doping purposes is far more complex.
Pre-workout drinks are quite popular now, and often claim numerous benefits such as "SKIN RIPPING PUMP" and "INCREASED TESTOSTERONE." More often than not, the only really effective ingredient in these drinks is the high doses of caffeine, which is an effective stimulant. The other stuff is almost always of dubious, at best, effectiveness. The ingredients purported to give a pump (i.e., increased blood flow and vascularity of muscle tissue), do no such thing, and any results people notice are likely because they're working harder due to ingesting huge amounts of caffeine. If you want to get hyped up before the gym, the cheapest solution is to drink a big cup of coffee. If you absolutely can't stand that, a reasonable substitute can be made with some sort of pre-workout loaded with caffeine and Gatorade mix.
A number of people use an ECA stack (ephedrine:caffeine:aspirin) in the order of 1:10:10 or 1:10:15 stack to lose weight faster, however, the effects are often small and not much evidence is out there to confirm that it can help you lose fat. It is banned in many countries because too much ephedrine explodes your heart.
Chemicals like clenbuterol and albuterol are quite popular weight loss drugs now, as they can be bought as grey-market "research chemicals" or through foreign "prescription" websites (While many of these are havens for bodybuilders, it is worth noting that both clenbuterol and albuterol have legitimate therapeutic uses, primarily in the treatment of asthma), have anti-catabolic and mildly anabolic (in layman's terms, muscle preserving and muscle building) properties and are even more powerful than ephedrine. These are powerful stimulants and come with the general side effects associated with such. Both are banned for any semi-serious sporting competition, and both can be very dangerous if mishandled. While dangerous, they do work as advertised, causing huge increases in metabolic function and quantifiable increases in physical endurance, albeit both at a price. Long-term use can easily nuke the thyroid, and overdoses can be lethal. The biggest problems with these sorts of things for bodybuilding or fitness use, however, is that once you go off, the weight comes back, because you're not teaching yourself how to lose weight in a safe, natural manner with diet and exercise, but rather resorting to dangerous chemical stimulants. This means the fat loss seen with these drugs is rarely permanent.
Taking creatine supplements does result in temporarily increased muscle size, but does not add any actual muscle mass. Creatine has this effect because it enables the muscles to retain more water, which leads to a temporary increase in size, an effect which goes away as the creatine is excreted. Creatine supplementation can also give small increases to anaerobic performance, allowing more intense workouts which in theory should accelerate weight and strength gains (see the "Effective" section). Most discussion and marketing of it are however unsupported woo.
On the illegal side as it has no approved use in healthy adults, human growth hormone (HGH) may be effective in achieving modest gains in muscle mass and reductions in body fat. There is little if any published evidence it otherwise improves physical performance.  Use as an "anti-aging" drug is pure off-label quackery and while benefits are uncertain the side effects across 31 high quality studies were common and annoying at best, potentially carcinogenic at worst. It is reputed to be a commonly used (and thus banned) substance in professional athletics, which raises its appeal. Any doctors assisting athletes doing this aren't publishing (or talking).
High dose anti-oxidant pills appear to be worse than useless. Yes, that's right, anti-oxidants hinder muscle strength. This appears to be because muscles require a certain level of oxidative stress in order to build over time.
Most of the other supplements might as well be as good as flushing your money down the toilet.
"Fat burners", which are widely available on the black and gray markets, are sometimes taken by bodybuilders looking for extra definition in the weeks before a show. They are often completely ineffective, but sometimes are both effective and potentially deadly. The danger of these differs from that of anabolic steroids, in that fat burners can kill through acute overdoses rather than as the result of chronic overuse.
2,4-dinitrophenol (DNP) is one of the most famous such substances. It works by increasing the proton-permeability of the mitochondrial membrane, thus uncoupling oxidative phosphorylation and limiting the body's ability to turn food into energy, forcing it in turn to burn through fat stores. Common side effects are fatigue and high fevers. DNP enjoyed a brief heyday as a slimmer in the 1930s before being banned after being linked to a number of cases of blindness. It is still used for industrial purposes, and drug dealers often acquire the stuff, package it in underground labs, and sell it to bodybuilders or people with eating disorders. In theory, if it were administered in carefully-titrated doses under controlled conditions, DNP might be safe; in practice, people often take it in high doses as part of "binge and purge" cycles, or else eat sugary carbohydrate-rich foods while on it under the assumption that the drug will prevent them from gaining weight. Overdoses lead to severe hyperthermia, sometimes as high as 43.1° C (109.6° F), tissue necrosis, and often, death.
Probably a bit less dangerous than DNP (and certainly less potent—notice a pattern here?) is yohimbine. It is a stimulant alkaloid derived from the bark of the yohimbe plant (Pausinystalia johimbe) of West Africa, where it is used as a folk remedy for erectile dysfunction. There is some clinical evidence that isolated yohimbine might help with fat loss in already-fit, highly-active individuals. All studies to date, however, have had serious limitations, such as: using small cohorts; only assessing professional athletes (read: hardly a representative sample); studying soccer players, i.e. people whose workouts are primarily aerobic rather than resistance-based; and/or analyzing yohimbine as just one component in a cocktail of purportedly-active ingredients. Side effects can include tremors, hypertension, racing heartbeat, irritability, and anxiety. As with DNP, yohimbine has been known to cause fatal overdoses. As such, it is restricted or outright banned in many jurisdictions, including much of the EU, Canada, New Zealand, Australia, Hong Kong, and Japan. In places where it is legal, be aware that supplements routinely contain far less or far more yohimbine than is claimed on the label.
At the opposite end of the "Dangerous, Stupid Stuff to Avoid Like the Plague" spectrum from fat burners lie site enhancement oils, the most common of which is called "synthol." Synthol is a cocktail consisting of 85% oil (normally medium-length MTC chains), 7.5% lidocaine, and 7.5% ethanol. It is a purely cosmetic doping agent taken by people who want to look bigger right now, dammit. Like anabolic steroids, synthol can give its user the appearance of bigger muscles. Unlike anabolic steroids (which do truly cause muscular hypertrophy), synthol gives the appearance of bigger muscles, and nothing but the appearance. Its method of administration is (unfortunately) so simple that any impatient fool with a syringe can figure it out: you just inject the oil directly into the muscle that you wish were bigger, and voilà, that muscle will look puffier. Synthol users gain no strength whatsoever from the drug, since it doesn't actually give you more muscle fibers, and may actually lose strength as the substance is known to cause nerve damage, scarification, oil-filled intramuscular cysts, and muscle fibrosis. Oh, and it also has been linked to multi-organ failure, pulmonary embolisms and strokes. As if all of that weren't bad enough, the aesthetic effects of synthol are short-lived and may actually be worse in the long run: because oil-enhanced muscles aren't actually stronger, they may start to droop over time, thanks to the combined effects of gravity and all of the dead weight they're carrying.
Anabolic steroids are highly effective at increasing muscle mass and performance but could have serious side effects if abused or if a course of PCT (post cycle therapy) is not followed. Steroids are the only substances that actually will do what most others claim. Some of the side effects of steroids include irritability, cystic acne, gynecomastia (breast enlargement) in men, and clitoral hypertrophy in women. They are also considered controlled substances, Schedule III in the United States. Besides being dangerous and illegal without a prescription, steroids are universally banned from anything besides bodybuilding magazine covers. The illegality of steroids also adds in other issues, as their black market nature means that ascertaining the authenticity and purity of the drugs is nearly impossible, and without the supervision of a doctor dosages are generally based around best guesses instead of hard science. A form of anabolic steroids (indeed, the same Barry Bonds was famously caught using) are commonly available via prescription for treatment for low testosterone, and in that role under strict medical supervision aren't terribly dangerous. While the loopholes are mostly closed now, this was a popular and safe way for athletes to legally take steroids before.
In general, protein powders are the most effective legal supplement, in that they do contain high levels of easily consumed protein and calories, which helps in building muscle. Protein (whey, casein, soy, etc.) powders are used for lots of other purposes besides bodybuilding. Mountain climbers carry them, since they are easy to pack and high in calories and protein for their weight. They're also used for the treatment of starvation, as starving people cannot usually digest solid food. However, the claims of these products are often wildly exaggerated, if not outright false. Simply taking a shake a day will not turn you into the Incredible Hulk, but they are cheaper than peanut butter or tuna; two scoops of powder can easily equal the same daily protein consumption of the average person, and a few additives (sugars) can be beneficial in adding quick calories post-workout. It's worth noting that protein powders are made of regular foods and their byproducts, and therefore the nutritional and physiological benefits are well understood. The same cannot be said about many other supplements. In the US, they are "regulated" by the FDA as "food," and are thus not required to demonstrate their safety before hitting the market. These products can be found in any health food store, and are often those stores' highest markup and biggest selling items. One company called Labdoor does independent testing though.
The most common workout booster additives are creatine (usually in monohydrate) and β-Alanine. Creatine is pretty well known as your body is constantly making the stuff to produce ATP for your muscles to power themselves. Having some pre-workout is not going to provide incredible strength, but for anaerobic repetitive workouts (weightlifting, strength training) it can add some 5% extra strength. It does not help with endurance in the slightest, making its use in cardio based exercises (such as running) wasteful. β-Alanine is more poorly understood, but can improve performance by reducing the rate of muscle exhaustion.
A more complicated booster are chemicals that boost nitric oxide production in the body. The result causes blood vessels to expand, increasing blood flow and allowing muscles to rip more easily. Since nitric oxide is a gas, you can't exactly huff this stuff and expect to perform (you will die from asphyxiation). HOWEVER, this can be dangerous if your blood pressure is already low, as it tends to lower it more because of the aforementioned blood vessel expansion.
Finally, just straight up caffeine is useful. A cup of coffee (with a a low calorie additive, if you wish) or diet soda before a workout isn't going to harm you and will give you more energy/make you feel less tired.
These products also include such things as electronic devices (sold on late night TV infomercials) purported to build muscle. Electrical stimulation is a valid therapy when used to prevent unused muscles from atrophying — such as can happen when a patient is immobilized and otherwise unable to exercise the muscle, but the devices seen advertised on TV will provide little more than minor cosmetic results. Adverts may show models with six-packs that could stop bullets, but they certainly did not get them from wearing an electrified corset. Any electrical stimulation device capable of providing substantial muscle mass and tone would require a painfully high and possibly dangerous electrical current. Also, a sensible diet and an exercise regimen kind of helps. Real bodybuilders don't go near these, but athletes who work out with a TV remote in one hand and in the other a bowl of Cheetos, are the intended market of these devices. From a scientific study on electrical muscle stimulation (EMS):
“”Companies claim that EMS is an easy and painless method for improving muscle strength, body composition, and appearance. These claims were unsubstantiated in the current study. Several factors, including the poor quality of the stimulator itself, probably combined to produce the results seen in this study. Subjects tolerated EMS amplitudes that produced discomfort but still were not able to achieve a muscular contraction of sufficient intensity to induce strength gains. Additionally, although manufacturers claim that workouts can be conducted quickly, workouts in the current study averaged 45 minutes in duration. Workouts using units with fewer stimulation channels would take even longer. Thus, EMS used under the conditions studied here does not appear to be a pain-free, quick method to increase muscular strength and is not recommended for the apparently healthy consumer.
There's always a plethora of new and exciting exercise devices hitting the market. While some may be effective, the question we must ask is "what can this do that weights or cardio can't?" The answer, more often than not, is nothing. Money is best spent on some good running shoes and a gym membership.
While these devices are nonsense, other simple measuring devices like heart rate monitors that are now found in most smart watches can be useful in measuring general levels of activity, reminding you to get up and do something and communications. On the other hand, these haven't been conclusively shown to be helpful in making you more fit.
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