A girl told me that she, on occasion, will skip her period by starting a new pack of birth control pills right after finishing another. (meaning, she doesn't give the week or so break in between to allow her period like is told by the doctor)
Now I have to admit, I have thought about doing this myself a few times! But I am wondering:
Is there any harmful side effects by skipping your period?

I've never heard of any harmful side effects. Break though bleeding is supposedly more common during the second month. And the few times I've done it when I did break for my period I had more cramps, headaches, I even got nausea. It wasn't severe or anything, just uncomfortable for a day or two.

I don't know about harmful side effects but I would think that the birth control value of taking the pills would be diminished if she does not follow the directions given by her doctor.

I'm just wondering whatyou mean by the birth control value diminishing? Are you saying you think they'd be less effective?

This is a very common thing women do; if you will be on vacation, or have a special event you simply continue on with the next pack. Sometimes cramping and bleeding will be worse when you get back on track, but as far as I know, there are no problems with doing this from time to time. My doctor told me there is nothing wrong with doing this on occasion.

There are pills designed to allow women to have four periods a year; I'm on one and I love it. These particular pills are FDA approved to be taken this way, and your doctor knows you take it this way. There is no loss of effectiveness, and no negative side effects that I'm aware of. The studies I've read and the doctors I have spoken to indicate there is no medical reason for a woman to have a period once a month, especially since when on the pill it's not a "real" period anyway. I've heard rumors about a pill you take non-stop (meaning NO periods a year) but I don't think it's made it to the market yet.