The male flowers of common hazel are grouped in pending yellowish catkins, which release pollen as early as January or February. Hazel pollen is highly allergic and cause early spring hay fever (sneezing, runny/stuffy nose and itchy eyes).
People allergic to common hazel are also allergic to other Betulaceae species, such as alder, hornbeam and birch. People with hazel pollen allergies may also experience oral allergy syndrome (itching/burning mouth) when eating raw nuts, apples, or other fruits.