Apparently “The Bitter Draught” is the name of painting by Adriaen Brouwer:
The Bitter Potion, also known as The Bitter Draught and The Bitter Tonic, was produced during the last years of Brouwer’s career and is among his most important works.
The sitter had obviously just tasted a bitter medicine and his extremely distorted facial expression is evidence of deep disgust. The bitter tonic was believed to have been made up of herbs from the cinchona tree which helped fight off malaria. It was also known as the ‘constipation of drugs’.
Source: http://www.artble.com/artists/adriaen_brouwer/paintings/the_bitter_potion
More about Adriaen Brouwer: https://en.wikipedia.org/wiki/Adriaen_Brouwer
I think the title could be referring to Jekyll’s serum.