Those with serious mental illness are often unable to work, and so do not have money to pay for housing. Low-income and supportive housing for the mentally ill is limited and it often takes years on a waiting list to get placed in a low-income apartment. Once homeless, it’s harder to take medications and go to doctor’s appointments regularly. This often leads to self-medication with alcohol and drugs.
Many homeless people also use drugs and alcohol to cope with the stress caused by homelessness, or became homeless because they need help with their substance abuse problem. People who are homeless are often arrested and housed in jails. Crimes are often related to homelessness: stealing, disturbing the peace, vagrancy, public intoxication, drug possession, prostitution or dealing drugs (in desperation for money to live on), and assault (in self-defense, or due to their mental illness or substance abuse). It would be much cheaper for the government to provide safe housing for people with mental illness rather than paying to keep them housed in jail for crimes related to homelessness.
It’s very dangerous to be homeless. The homeless are often robbed, beaten up and raped. People with mental illness are often too sick and vulnerable to defend themselves. This is why people with mental illness are more likely to be the victim of violence than the average person.