If you take eucalyptus oil by mouth you should dilute it first with some water because it could be poisoning if taken undiluted.
Also if you do intake too much eucalyptus oil then it could be bad and lead to eucalyptus poisoning which include symptoms such as stomach pain and burning, dizziness, muscle weakness, small eye pupils, feelings of suffocation.
If you think you've been poisoned or taken too much eucalyptus oil then you should go to the emergency room or urgent care just to be safe.
Using eucalyptus oil in moderation is healthy and safe but ingesting too much eucalyptus oil can be bad and potentially deadly.
Also inhaling too much eucalyptus oil could be bad as well and could get you high or make you sick but once again when eucalyptus oil is used in moderation it's safe and healthy.
If you swallow eucalyptus oil undiluted as well it can cause seizures.
If you take 3.5 mL of undiluted eucalyptus oil without first diluting it with water it can be fatal.