Liquid nicotine used in e-cigarettes can be poisonous, but is not regulated by federal authorities, The New York Times reports.

The liquid is extracted from tobacco and mixed with flavorings, colorings and chemicals. Tiny amounts, either ingested or absorbed through the skin, can cause seizures and vomiting. It can even be deadly, the newspaper notes.

These e-liquids are sold legally in stores and online. People keep them in small bottles to refill their e-cigarettes. Children may be attracted by the liquids’ bright colors and flavors such as bubble gum, cherry or chocolate. “It’s not a matter of if a child will be seriously poisoned or killed—It’s a matter of when,” said Lee Cantrell, Director of the San Diego Division of the California Poison Control System. Dr. Cantrell said e-liquids are much more dangerous than tobacco, because the liquid is more quickly absorbed.

The number of calls to poison control centers linked to e-liquids rose to 1,351 last year—a 300 percent jump from the previous year. That number is likely to double this year, according to information from the National Poison Data System.

Many people who use the liquids don’t realize the risks, according to Ashley Webb, director of the Kentucky Regional Poison Control Center at Kosair Children’s Hospital.

Most e-liquids contain between 1.8 percent and 2.4 percent nicotine. These levels can cause sickness in children, but are unlikely to be lethal. E-liquids containing higher nicotine concentrations, from 7.2 to 10 percent, are widely available online. A lethal dose at such levels would take “less than a tablespoon,” Dr. Cantrell said. “Not just a kid. One tablespoon could kill an adult.”