Contains a range of different sugars, aromatic oils, also pollens and bee proteins. Royal jelly and beeswax related, of course.
High fructose content can cause GI intolerance in some.
Allergic reactions can happen, often unrecognised, mostly due to specific pollens (depending on what flowers the bees feed on), in minority to bee proteins. Commercial honey tends to contain v low amounts of pollen, due to production techniques. IgE test for honey is available, but you may need to skin prick test with the specific honey if negative.
Honey eaten all year round is rumoured to prevent hay fever symptoms because of the pollens it contains, but this has not been proven, although it’s a nice idea related to immunotherapy. Depends on getting the right pollens of course – bees don’t like grass and birch flowers, probably. In some it may just trigger allergy symptoms.
Cross reaction between honey and bee venom is reported, not surprisingly, but not automatic.
Plant toxins can be present in sufficient quantities in honey to cause poisoning eg rhododendrons (some species).
Botulism reported in infants – failure to thrive, hypotonia, cranial nerve palsies. Clostridium and other bacteria cannot grow in honey due to the high sugar content, but spores can be present. So advice is not to give honey to infants.