One in five American seniors suffers from depression. The illness can be crippling at any age, but it wreaks an especially harrowing toll among the elderly.

Fortunately, 80 percent of those who obtain medical care for depression will see their symptoms improve, and in many cases disappear altogether. With the prevalence of depression among seniors twice as high as in the general population, it’s critical for older Americans to have access to adequate treatment.

Yet the Centers for Medicare and Medicaid Services (CMS) recently proposed a rule change that would have made it harder for seniors to get the medications they need. The sharp reversal of policy was unwise both clinically and on cost-containment grounds.

The change concerned the Medicare Part D prescription drug benefit. Thankfully, Washington lawmakers were able to halt the proposal.

But lawmakers must remain vigilant to ensure that seniors dealing with depression and other devastating illnesses continue to have access to the prescription drugs they need — as the same proposal could easily resurface in the coming months.

The proposed rule would have eroded a long-standing policy guaranteeing seniors access to medications considered vital to their health. Currently, insurers are required to cover “all or substantially all” drugs in six “protected classes” that pertain to treatments for depression, seizures, autoimmune disorders, transplant rejection, cancer, and HIV.

But, with an eye to the short-term bottom line, CMS had proposed stripping antidepressants, antipsychotics, and immunosuppressants for transplant rejection of their protected status. If adopted, insurers would no longer have had to cover the full range of therapies available. Seniors would have seen their treatment options reduced and could have even lost access to medications they’ve long taken.

The CMS even acknowledged that seniors and their doctors would have had less options in finding effective treatments. According to its own analysis, patients who currently have access to 57 mental health drugs could have seen their options dwindle to 15. Those who rely on antipsychotics, another affected class of medicines, could have seen their options slashed by 12. The agency defended reduced choice on the grounds of reducing costs.

Not surprisingly, physicians and patient advocates took a different view. They pointed to the negative impact such a change would have had on the well-being of many patients, a consideration difficult to capture solely in budget terms.

Doctors have long observed that different patients respond in different ways to the same medication, especially for complex illnesses like schizophrenia and depression. According to the largest trial of multiple medication treatment for depression, conducted by the National Institute of Mental Health, only half of the participants responded to the first treatment they received. The other half had to try up to four different treatment strategies involving multiple medication regimens before they found relief.

Not having access to a full range of medicines can have profound health consequences. Those suffering from mental illness live on average 25 years less than other Americans, due in large part to complications from treatable chronic medical conditions.

In the case of depression, studies have found that depression among the elderly nearly triples the risk of stroke. Depressed seniors also have more difficulty recovering from heart attacks, hip fractures, and such infectious diseases as pneumonia.

For the 2.4 million American adults who suffer from schizophrenia, effective treatment is critical. Almost half of those diagnosed with the illness attempt suicide. But with appropriate medication, that risk diminishes significantly. In fact, nearly 50 percent of patients experience positive outcomes with treatment.

Restricting patient choice does not provide effective care, nor does it control costs. Switching patients to cheaper drugs may have saved money in the short term, but the cost of dealing with poorer health outcomes would soon more than offset any such savings. As one medical journal puts it, “Nothing is more expensive than treatment failure.”

This is especially true for some of Medicare’s most vulnerable patients. A full 83 percent of Part D prescriptions for antipsychotics and 51 percent for antidepressants are filled by low-income seniors. These seniors are not only in poorer health, they’re particularly susceptible to treatment disruption. When they can’t take their medicines as prescribed, they often end up in need of inpatient care, at huge cost to the system.

Medicare Part D has been one of the government’s most successful healthcare programs, boasting 90 percent participation and a 94 percent approval rating. The program already has effective tools to manage the use of drugs within protected classes and keep costs down.

While Part D is safe for now, lawmakers must remain ready to defend it.

Charles S. Ingoglia is the senior vice president of public policy and practice improvement for the National Council for Community Behavioral Health, the unifying voice of America’s community mental health and substance use treatment organizations.