The Overdose is a primary weapon for the Medic. This weapon has a 10% damage reduction, but retains the same reload and firing times as the default Syringe Gun, so needles at point blank range do 9-10 damage instead. The main feature is that for every 10%, the medic gains 1% of speed making a medic with 100% Ubercharge move 10% faster, so s/he will move at a speed of 117% speed. This weapon is good if you for being safe if you encounter a enemy you cannot kill easily with your current weapons and have 100% ubercharge, this will let you run away easily. The damage reduction only makes your syringe needles do about 1 damage less each, so it is somewhat ignorable.
The Overdose has the same black and grey frame as the stock Syringe Gun, and the air compressor at the back is connected with orange tubing to the barrel. A red box is on top of the frame, presumably where the syringes are that the gun fires, and the orange tubing is taped to the box with duct tape.