Items are used to modify aspects of the monsters' growth, and they can sometimes influence the transformation of the monster. To use the same item on multiple monsters, you'll need to buy them from the shop at the top of the tower.
Speeds up time to twice its normal speed. Time flies while watching TV. Because of this, monsters age faster, regenerate hp faster, get hungry faster, etc...
Makes your monsters... Well, lose weight by pooping more often. Useful to make overweight monsters last longer, as it removes the overweight penalty as soon as the monster poops.
Improves the monster's HP regeneration by x1.5. Doesn't affect HP recovery from potions. Very useful to reach high levels with your monsters.
Food becomes irrelevant for the monster using it. The stat will stay frozen until it's removed. Monster can be fed normally, but the numbers will never go down. If the monster is at 0 when using it, it will still suffer from starvation. Additionally, the monster will not loose weight either.
Freezes monsters in time, stopping their aging, attacks, hp regeneration, everything. Also very important to note is that any heroes that appear when even one of your monsters is using it, will NOT drop a single potion until all hourglasses are removed, most likely to avoid farming levels without aging.
Unhealthy. Despite its description, however, doesn't affect health. It will only make that monster age faster, very useful for when you want to rush aging steps, or free space in your tower. The aging process will add 10 seconds per second to the current aging time. So, a normal monster would have 1+10 for a total of 11 seconds off the monster's life timer per second. Additionally, Overweight ads an additional +3 seconds per second, so an overweight monster with a toadstool will age by 14 seconds per second (1+10+3).
Increases the POW stat of a single monster by 20%, therefore granting extra gold by that same percentage when attacking towns. This boost does not affect challenge mode.
Increases the INT stat of a single monster by 20%, therefore granting an extra boost to the total INT of your tower - thanks to these you can affect what type of Heroes appear (and what monsters they drop). This boost does not affect tower challenges.