If you combine ingredients which share both positive and negative effects, it always creates a potion from it, and minimizes the negative effect. This means, for example, in Oblivion, I couldn't combine Steel Blue Entoloma with Fire Salts to create Fire Damage poisons, because they also both had the beneficial effects of restore magicka and resist frost; that's just one example, but it's frustrating because Fire Damage is a kind of rare effect - I think there's only 4 ingredients in the base game which have that effect, and 2 of them cannot be combined; there's many other examples of this situation. If my character is a skilled alchemist, and if he can formulate potions to minimize negative effects, it seems logically that he should be able to do the opposite as well: combine the ingredients in a formulation which is a poison which maximizes the negative effects, and also has a minor beneficial effect. All you'd really need is in the alchemy window, add a toggle button which switches between poison and potion. When clicked, it would do a couple things - if you select to make a poison, it should change the auto-name to the first NEGATIVE effect, instead of the first positive effect, change the strengths of the different effects so that you get larger negative effects and smaller positive effects, and of course, flag the created item as a poison so the game knows to not have your character drink it when you equip it in the inventory, but instead apply it to the weapon.
It might be too late for the devs to incorporate this in TES V, or perhaps they've already done this (I mean, who knows, right? I imagine I can't be the first player to ever request this), but I hope it makes it into TES V.