Seville orange, bitter orange, sour orange, or marmalade orange refers to a citrus tree (Citrus × aurantium) and its fruit.
If your medication requires you to avoid grapefruit juice, it is also recommended you should also avoid Seville orange juice.
Seville orange juice tastes closer to lemon juice than sweet orange juice. I would substitute 3 parts lemon juice to one part orange if you can’t get bitter oranges.
Clemintine, Seville orange, and a regular orange
Seville oranges are not as pulpy as the modern orange.
With a sample size of 1, I got 1/4 cup of juice and about 1/3 cup of seeds from one Seville orange.
With a sample size of 1, I got almost 2/3 cup of juice from a regular orange.