The injectable form of the inexpensive drug methotrexate is used to treat Acute Lymphoblastic Leukemia, or ALL.
"Obviously, it's very frustrating being a parent and not being able to get a hold of the medication that they know they need to be on," said James Wiernas, of Dougherty's Pharmacy in Dallas, Texas.
One of the nation's largest suppliers of the drug suspended operations back in November at the plant that makes it.
Now, the nationwide supply of it may run out within two weeks.
"This is not something that you can start and stop whenever you casually want," Wiernas said.
Nearly 3,600 children and teenagers are diagnosed with ALL each year.
Doctors say about 90 percent can be cured.
The cupboard is bare at Dougherty's and other pharmacies.
"A month or two, we've been completely out," Wiernas said. "Then we'll get some. As soon as we do, more often than not, it goes off our shelves pretty quickly."
The supply is becoming so tight that pharmacists at Dougherty's even heard from a doctor at Parkland Hospital who was looking for the drug.
"It's definitely coming to a head now," Wiernas said.
The Food and Drug Administration and others are urging companies to increase production, but no one can force them.