Fillers can be derived from any starch source including corn, potatoes, tapioca, and wheat.
Ingredients to Look For
Unfortunately, very few medications are clearly labeled as gluten-free.
Cross-contamination during the manufacturing process is another potential problem.

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Best Practices for Avoiding Hidden Gluten
Tell the pharmacist that your medications must be gluten-free.
Remind them every time you have a prescription filled.
They would need to look up this information from the manufacturer.
When generic medications are available, your insurance company will probably not approve the brand-name drug.
Generic medications, however, are not required to contain the same fillers as the brand name formulation.
Just because you’ve confirmed that a brand-name medicine is gluten-free doesn’t mean the generic form is safe.
Likewise, one generic being gluten-free doesn’t mean all generics are.
Few drug companies have a policy of avoiding gluten completely.
Ask to be given a second-choice prescription in case the first-choice medicine turns out to be unsafe.
The Celiac Foundation.Gluten in Medications, Vitamins, and Supplements.
National Celiac Association.Do I need to check that my medication is gluten-free?