Malady, Wits End and Madred's BloodRazor all stack with Teemo's Toxic Shots. This means that your basic attacks deal a lot more magic damage than physical damage, however, he still does decent physical damage. This makes him very hard to build against as he deals large amounts of both damages. The Frozen Mallet and Hextex Gunblade give him much needed sustain. Frozen Mallet along with Boots of Swiftness, your W and Flash turn you into an unstoppable chaser. Rabadon's DeathCap, Malady, Boots of Swiftness and Madred's Bloodrazor are the core of your build. You must then choose between Wit's End, Hextech Gunblade and Frozen Mallet.

1. Frozen Mallet
i) Large amounts of HP.
ii) Slow allows for chasing and escaping.
iii) Contributes to physical damage output.

2. Hextech Gunblade
i) Increases magic and physical damage output.
ii) Spell vamp. and lifesteal give sustain.
iii) The Active is very useful for chasing or for adding to your burst.

3. Wit's End
i) Increases attack speed (by a lot).
ii) Increases magic damage on hit (passive).
iii) Gives some magic resist if they have a AP heavy team.

I take Flash as it is useful for getting out of tight situations before you have Frozen Mallet and Hextex Gunblade for sustainability. Ignite stacks with your poison allowing you to pick up kills early game.

Alternative runes are flat AP or AP p/level runes as all of your powers scale off AP.

If you want to, you can lose the Armour Pen. and Life Steal masteries and take Blast and ArchMage instead.

I max Toxic Shot first as On-Hit-Effect Teemo revolves around it. 1 point in Sprint combined with your Boots mean that you are faster than anyone except someone with Boots of Mobility and of course Hecarim. Max Blinding Dart 2nd because it contributes a lot to your burst damage and it lets you survive ADC and Fighters long enough to kill them...Hopefully.