What kind of Medicare do you have? Before seeking treatment, it is important that you understand what coverage you receive. You can check your enrollment. Most users are covered by Original Medicare, but some are covered by Medicare Advantage, a type of private health plan.
Do you have Medigap and what will it cover? Medigap is supplemental insurance sold through a private company that covers the gaps in coverage that exist with Medicare. Medicare Matters explains these coverage gaps and how Medigap works.

Do you need inpatient or outpatient treatment? Your plan will cover both inpatient and outpatient addiction treatment. Inpatient treatment is covered by Part A and outpatient treatment is covered by Part B up to 80 percent.
Do you meet the criteria for coverage? As this article from LiveStrong notes, certain criteria must be met in order for your inpatient drug addiction treatment to be covered:

The treatment facility must participate in Medicare.
Your doctor must indicate that it is “medically necessary” and create a treatment plan.
Medicare must agree that the it is reasonable and necessary.
How do you find a facility that accepts Medicare? The Substance Abuse and Mental Health Services Administration provides a Treatment Services Locator. Type in your City, State or zipcode to see a map of the facilities in your area. The facilities will also be listed on the right-hand side of your screen. Click on “More Information” to find out if a specific facility accepts Medicare.
What isn’t covered? This Coverage Summary explains Medicare’s “limitations and exclusions.” For example, the program won’t pay for procedures or drugs that are still being studied.

What should you do if your addiction treatment is denied? According to the Center for Medicare Advocacy, lack of direct supervision by a doctor is one common reason addiction treatment claims are denied. You can appeal these decisions. The AARP provides information on how to do so.