Co-sleeping is a catch-all term that needs to be broken down into subcategories. Sharing a room and sharing a bed are quite different practices.
The American Academy of Pediatricians (AAP) recognizes this distinction. It advises that infants should always sleep on their backs and on a firm surface, and on the issue of co-sleeping, recommends that "the baby should sleep in the same room as the parents, but not in the same bed (room-sharing without bed-sharing)." The idea is that room-sharing embraces the benefit of close monitoring of the baby but avoids any risk of a parent inadvertently rolling over on the child.
Without a doubt, alcohol, tobacco, or other drug use by the parent, or low birth weight in the infant, make bed-sharing too risky.