Although low self-esteem is not categorized as a mental health condition, there are clear links between the way we feel about ourselves and our overall mental and emotional wellbeing. Someone with low self-esteem has negative feelings about themselves, believing that they are not worthy of love, happiness or success.
As humans, we strive to interact with others and the relationships we have with those closest to us help define us as people. So negative relationships ultimately equal negative feelings and a negative perception of ourselves.
Also, low self-esteem in childhood and early adulthood can be a predisposition to addiction in later life. Many addicts use substances such as drugs or alcohol to help ease the negative feelings they have about themselves. But over time this method of escapism develops into an addiction which has detrimental effects on their already depleted self-esteem levels.
Lastly, low self-esteem tends to work in a vicious cycle with other mental health conditions like depression and anxiety. It's hard to say which comes first, only that the combination is both common and troublesome. Someone who already lives with a mental illness may find that low self-esteem can develop due to the social stigma surrounding mental illness. Stigma can perpetuate the feeling that they have somehow failed.