J.K. Lasser's Your Income Tax 2013: For Preparing Your 2012 Tax Return

17.7 Expenses of Your Dependents

You may deduct your payment of medical bills for your children or other dependents, subject to the AGI floor (17.1). You may deduct the expenses of a person who was your dependent (21.1) either at the time the medical services were provided or at the time you paid the expenses.

In determining dependent status for medical expense purposes, some of the regular exemption tests for dependents (21.1) can be disregarded. If you are unable to claim someone as your dependent for one of the following reasons, you may deduct your payment of medical costs on their behalf: (1) the person is your child who is claimed as a dependent by the other parent under the special rules (21.7) for divorced/separated parents; see below, (2) the person has gross income exceeding the limit for qualifying relatives ($3,800 for 2012), (3) the person files a joint return with their spouse, or (4) you are the dependent of another taxpayer and thus are barred from claiming any dependents on your return. Such a qualifying person must be a U.S. citizen or national, or a resident of the United States, Canada, or Mexico, unless he or she is an adopted child who lives with you. See Examples 1–3 below. A child may not deduct medical expenses paid with his or her parent’s welfare payments; see Example 4 below.

Divorced and separated parents.

You may be able to deduct your payment of your child’s medical costs, even though your ex-spouse is entitled to claim the child as a dependent