New Jersey's expansion of Medicaid means that thousands of low-income residents will be eligible for mental-health services and treatment for drug and alcohol addiction. That's the good news.

The bad news is that many thousands more, those who are already eligible for Medicaid, will not get that expanded coverage.

As many as 104,000 adults are expected to be enrolled in the Medicaid expansion beginning Oct. 1. Under the Affordable Care Act, these new enrollees will be eligible for nonmedical detoxification, outpatient services for substance abuse, short-term residential and halfway-house services, as well as treatment for psychiatric emergencies.

But the more than 1 million residents currently enrolled in the state's Medicaid program, called New Jersey FamilyCare, will not have access to those services.

So, while this is a step forward in dealing with two of the most difficult health issues we face - mental illness and addiction - it is not as big a step as we need.

Just last week, the nation got another shocking reminder of the terrible cost of mental illness when a disturbed gunman killed 12 people at the Washington Navy Yard.

These too-frequent incidents of mass violence may be the most visible and shocking cost of mental illness, but they are not the only cost. Each day, many families deal with loved ones who are battling the effects and the stigma of mental illness and with a health care system that is expensive, challenging to navigate and often simply inadequate.

Others face the seemingly intractable problem of drug or alcohol addiction. Addiction fuels crime, and drug overdoses are now the leading cause of accidental deaths in the United States. Yet treatment options are limited, even for those with good health insurance.

For the state's poor, those challenges are multiplied. And any effort to provide more services for them requires finding money that the state doesn't have.

But while the federal government pays half of most Medicaid services, it will pay 100 percent of the costs for these newly eligible recipients for the first three years. By 2020, it will still be paying 90 percent.

So this program can be a laboratory of sorts, allowing state officials to see what difference offering these services can make, and providing information for advocates who want to expand this coverage to other low-income residents.