Wiki:
Eucalyptol is an ingredient in many brands of mouthwash and cough suppressant. It controls airway mucus hypersecretion and asthma via anti-inflammatory cytokine inhibition. Eucalyptol is an effective treatment for nonpurulent rhinosinusitis. Research showed that treated subjects experienced less headache on bending, frontal headache, sensitivity of pressure points of trigeminal nerve, impairment of general condition, nasal obstruction, and rhinological secretion. Side effects from treatment were minimal.
Eucalyptol reduces inflammation and pain when applied topically. It kills leukaemia cells in vitro.
In higher than normal doses eucalyptol is hazardous via ingestion, skin contact or inhalation. It can have acute health effects on behaviour, respiratory tract and nervous system. The acute oral toxicity (LD50) is 2480 mg/kg(rat). It is classified as a reproductive toxin for males and females.
Cineole based eucalyptus oil is used as a flavouring at low levels (0.002%) in various products.
----
Most Eucalyptus flavoring is oil based so you just can't add it to juice on DIY. It is, of course, a common flavor in cough drops, Hall's, eg. but always used at the low levels - 0.002 % is very small. You wouldn't want to do even one drop in a 10ml bottle even, (0.005%) if you were to find a water soluble version.