Most private health plans cover addiction and mental health treatment. Federal parity law is the reason. It requires insurers to treat substance use and mental health care the same way they treat any other medical condition, so coverage for rehab usually exists on your plan already. The open question is not whether you have coverage. It is which levels of care your plan approves (detox, residential, outpatient) and what your share of the cost looks like once deductibles and copays are figured in.