Vaping may be as hard on the lungs as conventional smoking, according to new research.
Flavorings in e-cigarettes may be particularly harmful to the lungs by triggering inflammation, scientists have warned.
Battery-powered vapes, like the Juul, are promoted as a tool to help smokers quit the habit.
But even short term use causes as much or more damage as the real thing, said the University of Athens team.
In experiments on mice they found the additives, including flavorings, caused lung inflammation similar to or worse than that seen in traditional cigarette use.
Study co-author Dr Constaninos Glynos said: 'The observed detrimental effects in the lung upon e-cigarette vapor exposure in animal models highlight the need for further investigation of safety and toxicity of these rapidly expanding devices worldwide.'
They simulate smoking a traditional cigarette by dispensing a vapor derived from liquid chemicals in a refillable cartridge that typically contain propylene glycol, nicotine and often flavorings.
Propylene glycol - a colorless and odorless additive - is found in numerous processed food and beverages. It is also used as a solvent in a number pharmaceutical drugs, the MailOnline reported.
The findings, published in the American Journal of Physiology-Lung Cellular, show e-cigs and refills are not well regulated - and their long-term effects on health are unknown.
So the researchers compared several groups of mice that received whole-body exposure to varying chemical combinations four times each day, with every session separated by 30-minute smoke-free intervals.
Dr Glynos said: 'Electronic cigarettes are advertised as a less harmful nicotine delivery system or as a new smoking cessation tool.
'Our findings suggest that exposure to e-cig vapor can trigger inflammatory responses and adversely affect respiratory system mechanics.
'In many cases, the added flavor in e-cigs exacerbated the detrimental effects of e-cig vapor.'
One group received cigarette smoke and three others e-cig vapors containing either propylene glycol, both this and nicotine or these two ingredient and a tobacco flavoring. A fifth batch just got normal, healthy air and acted as a control.
Some animals in each party underwent the regime for three days (short term) and others four weeks (long term).
There was an increase in markers of inflammation, mucus production and altered lung function in all three e-cig groups - after only three days.
But those getting propylene glycol alone showed fewer negative effects with long-term exposure.
Dr Glynos, of the University of Athens, said this suggests the additive elicits only a temporary irritation that eventually subsides with continued use.
In addition, two inflammation-producing proteins became elevated in the flavoring group.
This means some of the many flavoring components on the market may not be safe for even short-term use, said Dr Glynos.
The condition of the e-cig groups alarmed the researchers. The level of oxidative stress - damage at a cellular level - in those exposed to flavorings was equal to or higher than that of the cigarette group.
However, respiratory mechanics were adversely affected only in mice exposed to cigarette smoke - and not to e-cig vapor after prolonged treatment.
Dr Glynos added: 'We conclude that both e-cig vaping and conventional cigarette smoking negatively impact lung biology.'
His findings follow a British study earlier this year that also suggested vaping is more harmful than previously thought.
It found some of the effects are similar to those seen in smokers and people with chronic lung disease - boosting the production of inflammatory chemicals in the body.
The University of Birmingham scientists extracted cells from lung samples provided by healthy non-smokers and exposed them to e-cig fluid, condensed vapor or nothing for 24 hours.
Exposure to the vapor increased cell death and the production of inflammatory chemicals.
And the ability of cells to engulf bacteria was significantly impaired in those exposed to vapor.
The Centers for Disease Control and Prevention says that e-cigs are safer than traditional cigarettes. And they suggest smokers should consider switching to vaping in a bid to help them quit.
But the agency also warns that the devices may cause lung disease, keep people hooked on nicotine, or act as route in to smoking for kids.