>>the best thing you can do is take it to a reptile vet. it could be some of the same reasons humans vomit, upset stomach, bacterial ifection, viral infection, etc. the only way to know for sure is through a vet. they will also get you the meds you need for it. it may help to actually get a sample of the vomit for the vet to analyze, if she gives you the opportunity. just check the phonebook and start calling vets, asking them if they do turtles. good luck and let us know how things go.
Don't forget to take a fecal sample with you to the vet too if you can get one.
Parasites can cause vomiting, but so can a lot of other things.
Reptiles shouldn't vomit so the poster is right, see a vet ASAP.
Good luck!
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PHRatz