Quite often, nursing mothers will wonder if it is safe to take cold medicine while breastfeeding. Generally speaking, there is little risk to your baby from taking over-the-counter remedies for a brief period of time. However, some drugs are safer than others; and a few can have a negative impact on milk production. The InfantRisk Center of Texas Tech University Health Sciences Center at Amarillo has a helpful analysis of common cough and cold remedies.
On a personal note, although I try to avoid all drugs while pregnant or nursing, if I am really miserable with a cold I will take Ibuprofen (Motrin, Advil). Not only does it relieve the pain of a headache, sore throat, etc., but its anti-inflammatory property will reduce swelling in mucous membranes such as your nose and sinuses, making it easier to breathe. And, according to InfantRisk, Ibuprofen is "the preferred analgesic in breastfeeding mothers as only small amounts get into milk".