Originally Posted by Lisa N

Caffeine is a stimulant but it has an effect on insulin which can cause problems in the long run. Whether it's helpful or harmful depends on the study you read; I've seen studies that 'prove' both. IMO a couple of cups of fully leaded Joe a day probably aren't going to make or break anyone's diet but too much isn't healthy, either.

Nicotine is also a stimulant and on average can raise metabolic rate as much as 10% but aside from the fact that the delivery devices for this drug are known without question to cause cancer (and lots of other nasty chronic conditions such as emphysema), it also contributes to insulin resistance and causes negative changes in cholesterol production (lowers HDL, raises LDL).
Whatever it may do in the way of speeding up metabolism is absolutely, positively, 100% NOT WORTH the cost that it exacts on your health for doing so. After all, it's so hard to enjoy being thin when you can't breathe.