Is It Safe to Reuse Unfinished Formula?

I just read your article on reusing unfinished breast milk. Have you found similar articles on reusing formula that our babies didn’t drink?
—Michelle

This is an evergreen question but also especially relevant at the moment, when throwing away formula seems like about the last thing you want to do. It’s one thing when it’s a small amount. But when you make an eight-ounce bottle and your child only drinks an ounce, the waste is real. Especially in the face of a shortage, it’s natural to ask whether you could save the formula, even for a little while.

CDC guidelines say no. This infographic clearly indicates that any unused formula should be disposed of immediately. The main concern is related to saliva. When an infant drinks from a bottle, saliva is mixed in with the formula. This can enhance bacterial growth, and such bacteria, consumed later, could make your child sick.

The question for the data is how significant this bacterial growth is. We can agree it is a problem in principle, but knowing how likely it is will be necessary to weigh this risk against other ones.

There are at least two recent academic papers that have tackled the issue. In the first of these, researchers had four healthy adult volunteers in their early 20s drink formula out of baby bottles. The researchers then took a sample of the formula for testing. They stored the bottles in the fridge and, three hours later, took another sample. Both samples were cultured for the presence of bacteria. What they found was that the infant formula bacteria resembled the bacterial makeup of saliva in both cases and, importantly, the bacterial load wasn’t notably different after three hours in the fridge.

A second paper took this a step further, with a larger sample size. In this case, 13 young adults drank the formula, and the authors tested the samples at the start, at three hours, at 12 hours, and at 24 hours. All samples were stored at refrigerator temperatures. The researchers found that the bacterial concentrations were indistinguishable at these time frames.

Putting these together, while it is true that the bacterial content of saliva infiltrates the formula, it does not (from these studies) look like the bacterial levels grow quickly over this period of hours. This would suggest — as the authors of these studies do — that partially used formula could be stored in the fridge and reused, even after an extended period.

These papers have limitations: small sample sizes, not pulling data from babies; and this is a place where more data would be helpful. But this preliminary evidence does suggest that concerns about bacterial growth due to saliva may be smaller than are sometimes perceived.