We do not recommend you serve formula from your sippy cup for two reasons.
Firstly, the straw has a no-spill valve and the formula may be too thick to pass through easily.
Secondly, formula can cause a build-up in the straw which may be difficult to clean thoroughly.
As always we recommend washing all parts of the cup/straws following each use using warm soapy water and the cleaning brush and/or on the top rack of the dishwasher.
Formula is fine to use with the teat, spout and the training cups in our transition value pack.