Most of the time, medical insurance premiums paid through payroll deduction are pre-tax, that means, already taken off your taxable income. Since the money is already taken off your taxable income, you can't include it as a tax deduction since that would be double-dipping.
Premiums you pay out of pocket with after-tax money are allowable to be listed as deductible medical expenses, although whether you will get an actual tax reduction depends on your overall tax deduction situation.
