Last year, about 250,000 teens who had never smoked before tried e-cigarettes, according to the US Centers for Disease Control and Prevention (CDC). That’s a shocking threefold increase since 2011. This trend is largely due to the fact that e-cigarettes have been marketed to the public as a “safe” alternative to traditional cigarettes.

There’s some truth to this claim. E-cigarettes don’t contain the harmful tar that regular cigarettes fill a smoker’s lungs with. In some cases, e-cigarettes have even helped people quit smoking. Those trying to quit switch from regular cigarettes to e-cigarettes, and then to e-cigarettes that only contain water vapor. Also, e-cigarettes, like traditional cigarettes, reduce one’s appetite and, since they’re a stimulant, can increase alertness.

However, like regular cigarettes, e-cigarettes are “addictive and habit forming, and… very toxic” according to warning labels on e-cigarette packages. Plus, they contain lung irritants that can cause allergic reactions and even trigger asthma attacks.

One of these irritants is nicotine, the highly addictive chemical that makes cigarettes so hard to quit. Nicotine causes inflammation, worsening allergy and asthma symptoms. E-cigarettes also contain tiny nanoparticles which “aggravate and inflame the mucus membranes in the lungs.” Plus, the flavors added to certain e-cigarettes can cause allergic reactions.

Though they may not be as bad as traditional cigarettes, e-cigarettes are still bad for your health and highly addictive, doctors warn. E-cigarettes can bring out asthma symptoms in those with mild asthma, and could even cause life threatening attacks in those with severe asthma.