Devices called vaporizers, which resemble large fountain pens, are quickly becoming a popular item among people using e-cigarettes, according to The Wall Street Journal. The refillable devices are less expensive and more potent than regular e-cigarettes.

Vaporizers are customizable, allowing smokers to put the devices together themselves, choosing their own refillable cartridges, heating coils and liquid drippers. Vaporizers hold more liquid than regular e-cigarettes. They produce bigger vapor clouds and last longer.

In an era when smoking has become stigmatized, some people say they prefer vaporizers because they don’t look like regular cigarettes.

According to Wells Fargo, vaporizers are growing twice as fast as e-cigarettes, and are approaching half of total sales of e-cigarettes. Vaporizers are sold online and at “vape” shops. The Smoke-Free Alternatives Trade Association estimates there are 16,000 vape shops in the United States, up from 10,000 last fall.

Poison control centers are reporting an increase in the number of calls they are receiving for nicotine poisoning from e-cigarettes. This February, centers received 215 calls, compared with about one per month in 2010. About half of calls related to nicotine poisoning from e-cigarettes involved children age 5 or younger. Vaporizers are considered more dangerous to children because they don’t have sealed cartridges, the newspaper notes.