What does laced mean anyways?
It means the two drugs were combined, with the implication being without the user's knowledge. For instance, cocaine is commonly laced with caffeine, in this case it is to bulk up the product (because caffeine is cheaper than coke) so if you sell 0.75g of coke mixed with 0.25g of caffeine the user thinks they're buying 1g of coke, but in reality they only bought 0.75g.
EDIT: wtf forget you wikipedia stole "bulk up" right from me