Ferns

Though bracken ferns are not frequently, if at all, used in cocktail ingredients, below are a few words of caution. 

According to the book California Foraging [link], bracken fern fiddleheads are carcinogenic and should never be eaten raw or after they have unfurled.  

According to the Centre for Food Safety (Hong Kong) [link]

• Avoid overindulgence in bracken fern products.
• Proper processing and preparation is essential in order to reduce the level of harmful chemicals in bracken fern. For example, boil the fiddleheads in large amount of boiling water for 15 minutes or steam the fiddleheads for 10 to 12 minutes or until tender and discard the water before further cooking can help to lower the level of water soluble toxic chemicals in the fiddleheads.
• The public is advised to maintain a balanced diet so as to avoid excessive exposure to natural toxins and contaminants in general from a small range of food items.

Further Reading:

"The Bracken Fern: A Natural Born Killer?" The Atlantic [link]