The glucose and fructose, etc. sold to food manufacturers, and the dextrose used in hospitals, are not chemically pure. The manufacturers obtain solutions that are 80% or more glucose (for instance), the remainders being residues from the source plant. Canada has strict laws about how much residue is allowed in the glucose, fructose, etc. used in foods (as explained in Part B Division 18 (pages 156–159) of Canada’s Food and Drug Regulations). In the case of corn-derived food additives, it is this corn residue that can cause allergic reactions in corn-sensitive people. It would be too expensive for the producers of corn-based sweeteners, thickeners, and so on to bother to remove these residues when the vast majority of people won’t notice the difference in sweetness or thickening power or have negative reactions to that material.
See “How to avoid corn in Canada” for more information and links about this issue.
Also, though it is an American site, the FDA clearly states in DailyMed that dextrose drips are contraindicated in patients allergic to corn. I personally have had a bad allergic reaction to a dextrose drip. It was not a pleasant experience.