Suggest Treatment For Headaches When On Miralax

My daughter is trying to drink the go-lytely as prescribed but every time she takes a sip she gets an instant splitting headache. She has been able to drink less than a liter of it over the past five hours. This is after taking ibuprofen for a headache an hour or so ago. She is 18 and had taken four doses of miralax in the 48 hours before her doctor prescribed the go-lytely.