Insurance premiums paid through payroll deduction for an employer's plan are almost always pre-tax and you can't take a deduction for something that is already not-taxable.
Premiums you pay out of pocket (such as Medicare part A, COBRA benefits, a Marketplace plan) can be included as itemized medical expenses if you itemize your deductions on schedule A.
If you are self-employed and you purchase medical insurance in your name or the name of the business, there is a small business health insurance premium deduction in the business interview.
