I know of three mods that will make Alchemy have an offensive application:
http://www.tesnexus.com/downloads/file.php?id=211: Drilling a hole in the end of arrows will allow the packing of ingredients to produce effects on impact.
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=3020: Allows you to brew 9 different types of thrown potions, capable of damaging, blinding, calming, paralyzing, silencing, or destroying the weapons of your opponents.
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=2129: This plugin adds the ability to brew poisons. Using these, you will be able to poison your arrows\bolts\throwing weapons.