The problem is the potential for giving kids the wrong dosage of medicine.
Major manufacturers recently agreed to make the drug in a single concentration to reduce the chance of dosage errors.
But, not all companies followed through, and products with different strengths remain on store shelves.
The FDA says, packaging is so similar, parents may not notice the new product has a different strength.
The agency stresses that the older products come with droppers, while the newer ones have an oral syringe for more precise doses.