I'd treat it a lot like drinking alcohol-- When I asked my MD about this, she said if I felt like drinking a glass of wine, etc. that I could do it immediately before or during a feeding, that way by the time the baby was hungry again it would be out of my system enough not to affect the baby. If you're sick and you're worried about it passing to the baby, just keep in mind that any antibodies you've developed for your cold, etc. are going to pass through your milk to the baby and help keep her from getting sick. If you need to take meds, just be conservative about them.