KNOXVILLE, Tenn. — There is a nationwide shortage of a crucial drug used to treat childhood cancers. 

Vincristine is a significant part of treating leukemia and brain tumors, among other diagnoses.

There used to be two manufacturers, now there's only one. 

RELATED: East Tennessee mom hands out special pajamas to kids fighting cancer

Take a tricycle ride with Noah Sileno, and you'd see--he's not afraid to use his voice.

"Very opinionated," his mother, Martha Sileno, said. "Lets everybody know if he's okay with what we're doing or not."

And what doctors have been doing, not every kid would be as positive about.

"Noah was diagnosed at three and half years old with cancer," Martha Sileno said.

B-Cell Acute Lymphoblastic Leukemia has forced the now four-and-a-half-year-old to rely on chemotherapy.

RELATED: Helping kids fight cancer comfortably

Part of his treatment includes Vincristine -- a drug doctors say a critical part of many types of cancer -- like Noah's.

But there's a problem.

Teva Pharmaceuticals recently decided not to produce the drug anymore, leaving Pfizer as the only manufacturer, causing a shortage nationwide.

"It's devastating enough to have a child with cancer, but then when you find out there could be some kind of chemo shortage, your fears are just astronomical," Martha Sileno said.

But doctors and staff at East Tennessee Children's Hospital said they have a three-month supply of Vincristine on hand.

Pharmacist Terry King is monitoring the shortage vigilantly.

RELATED: Children's cancer treatment drug shortage could impact your pets

"We changed the way we mixed this drug, in that we would instead of preparing the drug in advance of the child arriving," King said. "We would wait until they got here, but we would do it so we could conserve the product we have."

Oncologist Dr. Shahid Malik said he hasn't had to address the shortage with his patients yet.

RELATED: Schools, hospitals prepare for potentially rough flu season

RELATED: Baby born to addicted mother confronts developmental challenges

"There is a mechanism in place--there are guidelines from the Children's Oncology Group for patients that are on study for and require this drug. (There are) ways to decrease the dose--miss some doses--but that is an unacceptable alternative at this point," Malik said. And so I truly hope that does not happen, but if it does, we do have guidelines in place that we would use."

King said children's is receiving another shipment of the drug in November.