How effective are enemas for constipation?

I am 27 years old and for the past one week I have been suffering from constipation. I will have to admit here that I am a smoker and I generally have 2-3 cigarettes every day. One of my friends told me to use enemas for constipation but as I have no idea about the enemas so I would like to know as to how effective are the enemas for constipation? Waiting for the replies!