Vapers who also smoke are twice as likely to suffer a stroke than adults who just stick to cigarettes, a study has warned.
Researchers analysed data on cigarette and e-cigarette use from 160,000 people in the US who were aged between 18 and 44.
Those who solely used e-cigarettes, and had never smoked, did not have an increased risk of stroke.
However, those who also lit up cigarettes were nearly three times more likely than non-smokers to have a stroke.
The findings suggest vaping is not a safe way of quitting tobacco if the person is still using cigarettes, and may be more harmful to the blood vessels, brain and heart, than if they had just stuck to cigarettes.
Smoking cigarettes alone is already known to be a key risk factor for stroke because it thickens the blood. Even those who have given up smoking and don't use any product at are still at an increased risk, the scientists said.
But research into how vaping may accelerates the chance of having a stroke is still in its infancy.
The academics cautioned more research is needed into the long-term effects of e-cigarette smoking amid an outbreak of vaping-related illness in the US.
While e-cigarettes were initially hailed as a healthy and effective way of quitting smoking, the case against them is mounting.
The research suggests that vaping may not be a safe way to stop smoking and when combined with cigarettes has an even worse effect on hearts, brains and blood vessels
The research was led by George Mason University in Virginia and published in the American Journal of Preventive Medicine.
Lead investigator Tarang Parekh, department of health administration and policy, at George Mason University in America, said: 'It's long been known that smoking cigarettes is among the most significant risk factors for stroke.
'Our study shows that young smokers who also use e-cigarettes put themselves at an even greater risk.
E-cigarettes have the potential to benefit some people, by helping them quit smoking. But scientists still have a lot to learn about whether e-cigarettes are truly effective for quitting smoking and what the long-term risks are.
Nicotine is already known to be highly addictive and harm adolescent brain development, which continues into the early to mid-20s.
E-cigarettes produce an aerosol by heating a liquid that usually contains nicotine. Aerosol is inhaled into the lungs and can contain potentially harmful substances, including heavy metals like lead, volatile organic compounds, and cancer-causing agents.
US health officials, the Centers for Disease Control and Prevention (CDC), are investigating an outbreak of e-cigarette or vaping, product use associated lung injury (EVALI).
The mystery illness has swept across the states. Officials have identified Vitamin E acetate as a chemical of concern. THC is present in most of the fluid samples collected from the lungs of ill people, and most patients report a history of using THC-containing e-cigarette, or vaping, products.
'Popcorn lung' is the nickname for bronchiolitis obliterans, a condition which damages the smallest airways in the lungs and has been linked to people with vaping-related breathing problems. However, there’s no good evidence that e-cigarettes could cause the lung condition, according to Cancer Research UK.
The flavourings in electronic cigarettes may damage blood vessels in the same way as heart disease, according to research published in June 2018.
The chemicals used to give the vapour flavours, such as cinnamon, strawberry and banana, can cause inflammation in cells in the arteries, veins and heart.
They cause the body to react in a way that mimics the early signs of heart disease, heart attacks or strokes, the study by Boston University found.
Other recent studies have also suggested smoking e-cigarettes could cause DNA mutations which lead to cancer, and enable pneumonia-causing bacteria to stick to the lungs easier.
Researchers at New York University subjected human bladder and lung cells to e-cigarette vapor, which is marketed as being healthier than tobacco.
They found the cells mutated and became cancerous much faster than expected and mice exposed to the vapour also suffered significant DNA damage.
In another study, scientists at Queen Mary University, London, found vaping makes users more likely to catch pneumonia – just like smoking tobacco or breathing in traffic fumes.
The vapour from e-cigarettes helps bacteria which cause the condition to stick to the cells that line the airways, they said.
The effect occurs with traditional cigarette smoke and those who are exposed to air pollution high in particulates from vehicle exhausts.
'This is an important message for young smokers who perceive e-cigarettes as less harmful and consider them a safer alternative.
'We have begun understanding the health impact of e-cigarettes and concomitant cigarette smoking, and it's not good.'
Academics used data from the 2016-2017 Behaviour Risk Factor