“”A physician who treats himself has a fool for a patient.
|—Dr. William Osler (1849-1919)|
Self-diagnosis, as the name would suggest, is when one attempts to diagnose oneself or family and friends when experiencing certain symptoms, typically with the help of symptom checkers found in books and, increasingly, the Internet.
Symptom checkers may be useful for finding what one should do (e.g. whether to go to a pharmacy, book an appointment with their doctor, call an ambulance, etc.), but in and of themselves they are useless and potentially dangerous as they may result in the individual using ineffective treatments whilst snubbing help which is effective. Alternatively, the nocebo effect may make the person's condition feel even worse, resulting in a feeling that they are in mortal danger and thus seek emergency help even though they don't need it. This of course results in people who genuinely are in mortal danger having to wait longer for treatment.
The problem with this is that even when the symptom checker one is using is from a trustworthy source, the vast majority of people have no medical training and thus may have difficulty noticing things that a professional doctor or nurse may notice (it doesn't help that some different diseases may have similar symptoms) and also may not have access to equipment which may help them determine the cause of the symptoms.
Psychiatric self-diagnosis (often incorrect) is popular, often focusing on mental health conditions which have attracted a certain amount of media attention in recent years, such as Asperger's syndrome, attention-deficit hyperactivity disorder, bipolar disorder, obsessive compulsive disorder, and various personality disorders. Often such self-diagnoses serve as a scapegoat for other problems with people's lives or behaviour. More far-fetched examples include self-diagnosing with multiple personality disorder, "otherkin" and "therian" conditions, and other purported phenomena such as indigo children. This tendency to over "diagnose" disorders tends to diminish the apparent legitimacy of mental health conditions which, of course, is of no help to people who are actually mentally disabled or ill.
Some people with are professionally diagnosed with these disorders are sympathetic to people who self diagnose. The expense of seeing a doctor in the US is so high that many undiagnosed people or people suffering from mysterious symptoms had to make do with the internet. Women have tales about struggling to get doctors to take their symptoms seriously. African Americans have huge issue with untreated and undiagnosed mental illnesses due to mental health issues being stigmatized in their communities. Self diagnosis on the internet can simply be a symptom of people desperate for answers.
Harassment over whether someone is faking an illness is an issue on social media. Professionally diagnosed autistic people can be harassed over the validity of their diagnosis if they defend self diagnosis.
 Comparison with Munchausen syndrome
People who self diagnose are accused of being attention-seeking snowflakes. It must be noted that people with Munchausen syndrome often claim they have a professional diagnosis instead of self diagnosis as professional diagnoses are taken more seriously. Common tactics used by people with Munchausen syndrome do not include: self diagnosis.
People who self diagnose do seem to be honest about the fact it was a self diagnosis.