After earning his medical degree from the University of Göttingen in 1841, he returned to Aurich, where he spent several years working as an optician.

Frerichs made many contributions to medical science, and is especially known for his research of kidney and liver diseases.

He published the first German textbook of nephrology, and performed microscopic research of Bright's disease.

Frerichs gave the first clinical description of progressive familial hepatolenticular degeneration (now known as Wilson's disease), and also discovered the presence of leucine and tyrosine in urine involving yellow atrophy of the liver.

Frerichs performed pioneer research of multiple sclerosis, and described nystagmus as a symptom of the disease.