Medicare Supplement Insurance, also known as Medigap, can generally be enrolled in at any time, but there are specific periods when it’s most advantageous to do so:

1. **Medigap Open Enrollment Period**: This is the six-month period that begins the first day of the month in which you are both 65 or older and enrolled in Medicare Part B. During this period, you can buy any Medigap policy available in your state without medical underwriting, meaning the insurance company cannot deny you coverage or charge you more due to pre-existing conditions.

2. **Outside the Open Enrollment Period**: You can still apply for Medigap, but the insurance company can use medical underwriting to decide whether to accept your application and how much to charge you. This means they can deny coverage or charge higher premiums based on your health status.

3. **Guaranteed Issue Rights**: In some situations, you have a right to buy a Medigap policy without medical underwriting, even if you’re outside the open enrollment period. This can happen if you lose certain types of health coverage, such as if your Medicare Advantage Plan leaves Medicare or stops providing care in your area, or if you move out of the plan’s service area.

In summary, while you can technically apply for a Medigap policy at any time, doing so outside your Medigap Open Enrollment Period or without guaranteed issue rights could lead to higher costs or denial of coverage.

Medicare Supplement Birthday-Rule.  If you are currently enrolled in a Medicare Supplement plan, you can change companies during your birthday month, without a medical review.  This open enrollment starts on your birthday and ends sixty days after your birthday.  This allows you to change companies with the same plan or one with less benefits, for example if you have a Plan G, you can move to another company plan G or downgrade to a plan N or other lower benefit plans.