eBay, Wish.com and AliExpress have announced that after a What? investigation they have found dozens for sale.
Our investigation looked at pills and powders containing yohimbine and synephrine — two ingredients that the Medicines and Healthcare Products Regulatory Agency (MHRA) says have "significant potential to cause harm if used without medical supervision or advice."
The substances are marketed as weight loss aids and workout enhancers, although there are doubts about their effectiveness.
More importantly, there are concerns about their health effects. The MHRA said in a statement that they "are capable of having significant physiological effects, particularly on blood pressure and heart rate."
What are yohimbine and synephrine?
Both yohimbine and synephrine are substances derived from certain herbs. They are used as ingredients in weight loss pills and pre-workout supplements.
Yohimbine is derived from the bark of a tree found in parts of Africa and has traditionally been marketed as a male aphrodisiac and dietary supplement for weight loss. Synephrine is a pharmacologically active compound derived from bitter orange extract.
Yohimbine is banned for sale in the UK except when sold by a pharmacist. However: who understands that it is not routinely available on prescription.
Regulating synephrine is more complicated than yohimbine, and the MHRA evaluates individual products containing the ingredient on a case-by-case basis.
However, since it can produce physiological effects, synephrine would normally also be considered a prescription drug and is therefore subject to strict quality, safety and efficacy controls.
An MHRA spokesman said it was "very likely that finished products containing either yohimbe or synephrine would meet the definition of a medicinal product and we have previously classified many such products as medicinal products".
The MHRA said it will "review" the products we uncovered in our investigation. "If we find products that qualify as medicines that do not have an appropriate approval, we take action," the spokesman added.
Freely available to UK consumers
Despite warnings about the safety of these ingredients and the fact that they're not to be routinely offered for sale in the UK, we were able to easily purchase a sample of nine products containing either yohimbine or synephrine, three from each platform.
Of the nine we bought, two didn't come with any health warnings or dosing information at all.
One had the name of the ingredient - in this case yohimbine - clearly visible on the packaging, despite being successfully delivered to a UK address.
Each platform had at least a dozen products for sale that contained yohimbine or synephrine at the time of our research. Prices varied considerably. Some items cost as little as £2.39 plus postage, while others cost over £80.
They could be shipped from countries like India, USA, Ukraine and Poland, with many of them offering free shipping.
eBay sales data, publicly available in product listings, showed that a sachet of synephrine powder claiming to offer "weight management" was purchased 197 times. A yohimbine product was purchased more than 146 times.
The data does not reveal where the customers who purchased the items live and there are no restrictions on the sale of yohimbine or synephrine in certain other countries, but we were able to easily purchase our own selections.
Are yohimbine and synephrine dangerous?
Both substances can cause serious harm, said Dr. James Coulson, a toxicology expert, told What? When asked if they might be dangerous, he said, "The answer is yes, to both." 
Coulson, a Cardiff University Reader and a practicing consultant with the Cardiff and Vale NHS Trust, is a member of the UK Committee on Toxicology and adviser to the UK National Poisons Information Service.
He has treated patients who have had complications from taking yohimbine and has addressed reports of complications from taking synephrine through the National Poisons Information Service.
Common symptoms of taking yohimbine or synephrine can include agitation, aggression, nausea, and increased heart rate and blood pressure, Coulson said. However, when taken in high doses, there can be "more dramatic manifestations [of symptoms]" affecting the heart and kidneys at times permanent damage and can lead to strokes and heart attacks.
"When it comes to these drugs, the critical implications that concern us are their effect on the sympathetic nervous system and particularly their effect on blood pressure," he said.
"If you took a lot of it, you would see the effects of secondary organ damage due to the increase in blood pressure [and] you would expect potential damage to the cardiovascular system. A lot of that depends on an individual's susceptibility. If you have one If you have underlying heart disease, it can certainly manifest itself in one of these episodes.'
Another concern of health professionals is that people who take supplements are often taking more than one ingredient at a time, unknowingly increasing their risk of worrisome side effects.
One reason for this is the perception that if something is plant-based, it is therefore harmless. "Some of the most potent poisons we know of occur naturally," Coulson said. "The ecosystem has been trying to kill us for years."
Studies have shown side effects
Both supplements are popular with some bodybuilders and gym-goers, but there have been a number of studies warning of possible side effects, while some consumers have expressed concerns about them on social media.
On a forum, one person claimed to have taken up to 300mg of yohimbine before working out and said the side effects were of such concern that they are considering going to the hospital. The person said they experienced excessive sweating and a " sky high” heart rate and blood pressure.
One of the yohimbe products we purchased recommended a dose of two 300mg capsules per day.
Studies have also been published in scientific journals warning of the potential side effects of yohimbine and synephrine, including a bodybuilder who required hospital intubation after a significant overdose of yohimbe, and links between synephrine and hypertension, cardiac arrhythmias, and myocardial infarction at larger doses.
We have filed a Freedom of Information Request with the MHRA Yellow Card program, which tracks reports from healthcare professionals and the public of adverse drug reactions.
The MHRA confirmed that it had received six reports of "suspected adverse drug reactions" to yohimbine and two reports of "suspected adverse drug reactions" to synephrine over the past decade means it was caused by the drug in question".
Similarly, it said in its reply, adverse drug reaction reports can be influenced by the "detectability" of the substance - meaning that not every suspected adverse reaction is flagged via the schedule.
Dietary supplements: know the facts
Taking a pill or supplement that claims to help you lose weight may sound tempting, but you can be bad for your health if you're not careful, especially if you don't know what's in them or where and how they're made.
The MHRA warns against self-medicating or self-prescribing. It says if you have a concern about your weight, consult your doctor or healthcare provider who is best able to assess your condition and needs judge.
Food supplements in the UK must include a list of ingredients, information on storage and dosage, and must not contain inaccurate health claims. Medicines are also subject to strict controls for quality and safety. Never buy without consulting a doctor.
Complex regulation
The regulation of slimming products is patchy and complex. Found which ones. Responsibility is shared between three government departments – the Food Standards Agency (FSA), the MHRA and the Department of Health and Social Care (DHSC) – as well as local authorities. 
None of the three regulators were initially able to provide clear guidance as to why there are concerns about yohimbine and synephrine, and which ones is aware that there is relatively little proactive oversight for these potentially unsafe ingredients sold online. This could expose consumers to potential harm.
An MHRA spokesman said it was "very likely that finished products containing either yohimbe or synephrine would meet the definition of a medicinal product and we have previously classified many such products as medicinal products".
The MHRA said it will "review" the products we uncovered in our investigation. "If we find products that qualify as medicines that do not have an appropriate approval, we take action," the spokesman added.
The FSA pointed us in the direction of DHSC when we contacted them, and a DHSC spokesman said in a statement: "Dietary supplements are strictly regulated and anyone selling banned substances as part of a food product will be prosecuted.
"The Medical and Health Regulatory Agency is investigating this issue."
How the platforms reacted
We reported our findings to eBay, Wish.com, and AliExpress. Each platform said they would withdraw the products after our investigation.
eBay said the items we tagged "have been blocked and removed from the eBay platform".
'eBay sellers must comply with the law [and are]... it is forbidden to offer prescription drugs, in accordance with the rules of the Medicines and Healthcare Products Regulatory Agency (MHRA). In addition to our own prevention teams and technologies, eBay works closely with them the MHRA and other regulatory agencies around the world, who directly report and remove listings of concern.'
Wish.com said, "In accordance with our merchant policies and terms of service, we generally restrict the sale of products that require a prescription or physician supervision.
“Sales of vitamins and dietary supplements are restricted to a limited number of pre-approved retailers in certain regions. Each such retailer is required to provide appropriate safety certifications and legitimate proof of authorization.
'The listings marked by Which? will be removed from our platform and we will continue to seek ways to actively prevent such products from being re-listed.'
AliExpress said the company takes customer safety "very seriously" and works hard to "ensure a safe shopping environment."
'The items marked by Which?were removed from the AliExpress platform as part of its investigation. Sellers who have listed these products will be penalized according to our policies.'
Which?requires action against unsafe products
The consumer advocate's latest investigation again raises serious concerns about the lack of controls and monitoring by online marketplaces and underscores the need for them to be legally responsible for unsafe products on their websites.
Responsibility currently lies with the third party suppliers on these websites, but there does not appear to be adequate enforcement action to prevent the sale of potentially unsafe dietary supplements and medicines to UK consumers. This could lead to worrying people being exposed to potentially harmful products .
Sue Davies, who?s head of consumer protection policy, said:
"It is worrying that our research found that these slimming supplements with potentially dangerous ingredients are readily available on online marketplaces. The limited regulation of these sites is not working - and that is leaving people exposed to harmful substances.
“Online marketplaces need to be given greater legal responsibility for the safety of the products sold on their websites so that buyers are much better protected.
"Regulators also need to be more proactive in monitoring potentially dangerous products for sale on these sites, which are growing in popularity."
