The absorption of other drugs may be delayed if taken at the same time as marshmallow. Marshmallow may also interfere with existing hyperglycemic therapy.

– Source: Pharmacy Times

Use caution if you are a diabetic.

Absorption into the body of other medicine may be delayed if taken at the same time as Marshmallow.
– Source: German Commission E Monongraph

Because it coats the lining of the stomach, marshmallow may interfere with the absorption of other drugs or herbs. To avoid any problems, take marshmallow several hours before or after taking other herbs or medications.
– Source: University of Maryland Medical Center.