I have successfully used these when I first became an LC for cup feeding.
Another LC had the same concerns you do about what they are made of and
expressed this to me. It made sense and I stopped using them.

Some of the families I work with have a lower income level and this was
something they had because they were taking pictures of their newborn. I have
since used many other cups, including a 1-2oz disposable cup you can buy by
the hundreds at places like Sam's Club.

Learning every day...
Annette

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