Coal tar is a thick dark liquid that is a by-product of deriving coke and coal gas from coal. Coal tar has both medical and industrial uses. Medicinally, it is a topical medication applied to skin to treat psoriasis and seborrheic dermatitis (dandruff). It may be used in combination with ultraviolet light therapy in a treatment called Goeckerman therapy, which is rarely used clinically, as of 2024.