(1) Background: “Patient health information” promote health literacy. “Patient guidelines” as a sub group reflect the current evidence about illnesses and treatment options adapted to the needs of laypersons. Little is known about factors promoting and hindering their use by people affected by mental illness and their relatives. (2) Methods: Telephone interviews (N = 15; n = 4 adults affected by mental illness, n = 5 relatives, n = 6 both applicable) were conducted according to the Sørensen model of health literacy. Data were recorded, transcribed and content-analyzed following Mayring. (3) Results: Health information is used regularly by individuals affected by mental illness and their relatives, but “patient guidelines” are largely unknown. Yet, there is a great willingness to use them. Main barriers are a lack of statistical knowledge, the complexity of health-related topics and cognitive impairment sometimes accompanying mental illnesses. Target group-oriented adaptation as well as transparent and even-handed presentation of (dis-)advantages of treatment options can increase trust. (4) Conclusions: Health information and guidelines can help affected persons and relatives to make treatment decisions by conveying unbiased, up-to-date knowledge. Target group-specific adaptations should be made for psychiatric illnesses and features specific to mental illnesses compared to physical illnesses should be included. Clinical practice guidelines must be distributed more widely to increase their impact.