“Dangerous” weight loss products containing substances that can induce a stroke or heart attack are being sold on websites such as eBay without any health warnings, an investigation has found.

The consumer group Which? found dozens of products on sale online containing plant extracts that can make users agitated or aggressive and increase their heart rate and blood pressure.

They all contained either yohimbine or synephrine, substances that the Medicines and Healthcare products Regulatory Agency (MHRA) has said have “considerable potential to cause harm if used without medical supervision or advice”.

Which? bought nine such products, which are used by some people seeking to lose weight, bodybuilders and gym-goers, through the online marketplaces eBay, Wish and AliExpress. Of those, two included no health warnings or dosage information.

Buyers could get the supplements sent to them from countries including India, the US, Ukraine and Poland at a cost of between GBP2.39 and GBP80, it found.

Dr James Coulson, a toxicology expert who sits on the UK’s Committee on Toxicity, warned that the extracts were dangerous and could damage users’ health.

“When it comes to these drugs the critical effects we are concerned about is their effect on the sympathetic nervous system and particularly their effect on blood pressure,” he said.

If someone ingested enough of the substances, “you’d then start to see the effects of secondary organ damage because of the rise in blood pressure [and] you’d expect to see potentially damage to the cardiovascular system”, he added.

Coulson is a lecturer at Cardiff University, a consultant in the NHS in Wales and a consultant to the UK National Poisons Information Service. He has treated people who have suffered side-effects after ingesting yohimbine and also handled reports of complications from use of synephrine.

He and other healthcare professionals are concerned that people may assume that herbal extracts are harmless because they occur naturally and not realise that they are potentially risky.

Sue Davies, the head of consumer protection policy at Which?, said: “It is concerning that our investigation has revealed these slimming supplements containing potentially dangerous ingredients are readily available on online marketplaces.

“The limited regulation of these sites is not working and that’s leaving people exposed to substances that can be harmful.”

The MHRA and other regulators “need to be more proactive in policing potentially dangerous products that are offered for sale on these sites”, she added.

Which? found a post on one online forum in which the person said they had ended up with “sky-high” pulse and blood pressure after taking 300mg of yohimbine before a gym workout and were so concerned by its impact that they considered going to hospital.

Ebay and AliExpress removed the listings for the products that the consumer group alerted them to, while Wish said it was in the process of doing so.

An eBay spokesperson said: “We take the safety of our users extremely seriously and work closely with the MHRA to help ensure sellers and listings on eBay comply with laws and regulations.”