Possibly. Here is what IRS Publication 538 says about the deductibility of prepaid expenses:
"Expense paid in advance. An expense you pay in advance
is deductible only in the year to which it applies, unless
the expense qualifies for the 12-month rule.
Under the 12-month rule, a taxpayer is not required to
capitalize amounts paid to create certain rights or benefits
for the taxpayer that do not extend beyond the earlier of
the following.
(1)12 months after the right or benefit begins, or
(2) The end of the tax year after the tax year in which payment
is made.
If you have not been applying the general rule (an expense
paid in advance is deductible only in the year to
which it applies) and/or the 12-month rule to the expenses
you paid in advance, you must obtain approval from the
IRS before using the general rule and/or the 12-month
rule."
So as long as your medical insurance prepayment is for not more than one year, it may be deductible in the year paid.
The last paragraph in the IRS information above means that if you paid any expenses in the past for a future year and did not elect to deduct them in the year paid under the 12-month rule, then you cannot apply the 12-month rule this year without getting prior approval from the IRS. However, if you never before paid any such expenses in advance, then no prior approval is required as you are making an initial election to use the 12-month rule.
Note however that this election does not just apply to medical insurance premiums, but to any prepaid item. So if you paid some other item in advance six years ago and did not elect the 12-month rule for that item, you cannot use the 12-month rule for this year's medical insurance premiums without applying for an accounting change through the IRS. You may have inadvertently done something without even realizing you did it. For example, paying your share of next year's property tax when you sold your home and taking the deduction in the year paid. This effectively elects the 12-month rule without making an overt election. Or not taking that same deduction until the next tax year, which effectively elects the general rule to the exclusion of the 12-month rule.
If the amount involved is significant, I would suggest you contact a qualified tax consultant to look at your specific situation.
