Well this is a bit of a yes and no question. Fat burners as a term defines everything from over the counter weight loss aids through to more illicit options. It’s supposed to just be applied to metabolism boosters, but the supplements industry being what it is, tends to muddle terminology a lot. We’ve even seen generic multivitamins claim to be fat burners, so you can see how the issue gets a bit muddled.
And to make this question a bit more confusing there have been companies which have included banned substances in supposedly safe over the counter fat burners. And there have been some cases whereas over the counter options seem to have had nasty side effects without having banned substances in them.
So, this is going to be quite a broad response.
Are Over The Counter Fat Burners Safe?
For the most part yes. With the exception of some mild caffeine side effects or some nausea most fat burning supplements aren’t going to do many people any serious harm. However, some do contain doses of ingredients far over the recommended intake range. Generally we recommend that people avoid proprietary blends and only use fat burners with transparent labelling, produced in either the US or EU (or generally western countries) as they generally have some requirements for 3rd party testing and organizations to help monitor safety. In the US this is the FDA’s responsibility to monitor supplements, but it’s not as if they’re incredibly stringent.
Now, there is of course always the risk of allergens and there are a few common ingredients which we tend to recommend people avoid, as whilst the ingredients are considered “safe for consumption” some have been known to be more risky than others.
For example, OxyElite Pro was implicated in 50 cases of acute liver failure. 
List of ingredients to avoid in fat burners
- Garcinia Cambogia – links to heart issues, on the FDA’s monitor list
- Synephrine – links to liver toxicity
- Caffeine – Obviously caffeine is fine in moderation, but just don’t take high caffeine fat burners with your morning coffee.
- Usnic Acid – links to liver toxicity
- Too Many Vitamins – Whilst humans can generally consume a lot more than the RDI of B Vitamins without any issues, many fat burners contain 100s of times the RDI, which is in theory fine, but energy drinks also do the same, so you can very easily over do it here and it’s something to watch out for.
Prescription Weight Loss Aid Safety
This is a totally different kettle of fish and we of course advise that you don’t take anything not prescribed. But, there will of course be more side effects from many of these. With the exception of perhaps something like orlistat which is available both over the counter and on prescription in several countries. Although it’s a fat binder rather than fat burner, and many other over the counter options would likely be more effective.
Risks of Buying Fat Burners From Asia or Latin America
One of the most significant concerns with fat burners is buying them from overseas and finding the presence of synthetic adulterants. These illicit substances, often added illegally to dietary supplements, can have severe adverse effects on liver health. Examples include sibutramine, fenfluramine, and rimonabant, which have been associated with cardiovascular complications and depression.  There have also been some cases of US manufacturers doing this too, but it's far less prevelant.
And Further Risks
The Food and Drug Administration (FDA) has found that some fat burners on the market contain dangerous substances. These products are often marketed as aids for fat loss but may contain untested and unstudied active ingredients. The lack of regulation and oversight makes it difficult to know exactly what is in these supplements and whether they are safe for consumption. 
So, Are Fat Burners Safe?
Mostly yes, but there are several untrustworthy products on the market. We do have a list of products here that we have seen the 3rd party testing certificates here.
That way we can be sure they’re safe as independent labs have tested the ingredients.
1 - https://www.ncbi.nlm.nih.gov/books/NBK548935/
2 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3076034/
3 - https://www.sciencedirect.com/science/article/abs/pii/S0278691518307907