"While 35 cases may not seem like much compared to the total number of people using e-cigarettes, we are nonetheless concerned by these reported cases," outgoing FDA Commissioner Scott Gottlieb and Principal Deputy Commissioner Amy Abernethy said in a statement.
For more on investing in health-care innovation, click here to join CNBC at our Healthy Returns Summit in New York City on May 21.
E-cigarette liquids contain high concentrations of the nicotine. If swallowed, they can be poisonous, often causing nausea, sweating, dizziness and tremors, according to the National Capital Poison Center. In severe cases, nicotine poisoning can cause seizures or even death.
The FDA said the evidence it has analyzed so far doesn't establish a clear pattern or cause for the cases.
"We want to be clear that we don't yet know if there's a direct relationship between the use of e-cigarettes and a risk of seizure," Gottlieb and Abernethy said.