If you sign up for a company health insurance, do you HAVE to pay for it for a full year?

Can you just drop it if you decide you don't want it within 6 months? What will happen if you don't pay?
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Once you sign up for it, you can only drop it during open enrollment, once a year, unless you die or leave the job. It comes out of your paycheck, so you don't have the option of not paying.
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