contraindicant

from The Century Dictionary.

• noun In medicine, a symptom or indication showing that a particular treatment or course of action which in other respects seems advisable ought not to be adopted.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Med.) Something, as a symptom, indicating that the usual mode of treatment is not to be followed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun medicine A contraindication.