One interesting (side?) effect of orgasms is that as one approaches orgasm, one's overall sensitivity decreases, and during orgasm it is almost impossible to feel any type of pain.

disclaimer: I have not conducted scientific studies to test this. All I know is from the personal experience of myself and several others.

It would appear that there is no pain that beats this pain killer. Toothaches, headaches, bruises, you name it... If the person in pain can reach sexual excitement, they can usually be brought to orgasm. At that moment they will feel no pain.

The evolutionary reasons for this seem rather obvious, at least for the male orgasm. When a male is nearing orgasm, which means that his genetic material is probably getting a good chance of continuing for at least one more generation, it would be a crying shame if he stepped on a thorn and had to stop. For the female, the situation is similar, although her orgasm is less important (evolutionarily speaking, of course. In human sexual encounters, the woman is MUCH more important, and should be revered and worshiped and brought to orgasm many times.)

But, there must be a balance. We would not want the couple, once lovemaking has started, to be completely oblivious to pain. So gradually, over the lovemaking, sensitization is decreased, until orgasm, when it is all but complete.

The down side: the second the orgasm is over - the pain returns, full blast. Now THIS is hell. Evolutionarily this makes sense, of course. But forget the post-orgasmic chill, forget fadings. The pain returns, and with a vengeance.

Knowing this, you should be now able to counter that well-known excuse:
Next time your SO says, "Not tonight, darling, I have a headache," you can safely say,
"That's alright. I know just the cure."

Also interesting:

The pain associated with menstrual cramps can be relieved by reaching orgasm (her, not you, doofus).

I know, I know: it sounds gross. However, if you're skilled at getting her off through clitoral stimulation by hand, it's really not gross at all...you don't even have to get near the troubled area.

If your girlfriend/spouse/SO/neighbor suffers from intense cramps, offer her this solution next time. If you do succeed in easing her pain, she'll be immensely grateful.

I'm sure you'll be able to think of some way for her to express her gratitude.

The pain-killing properties are an extremely useful effect of sexual excitement - and not just orgasm, either.

Several months ago, I was injured at work while assisting a client who was about to fall. This royally screwed up my back and neck. I have been in a near-constant state of pain since the injury, with a few notable exceptions.

Once I was able to have sex again, the most fun of the exceptions became when my beloved and I were having sex. The endorphins that are released during sexual arousal do a wonderful job of combating the pain and muscle spasms and cramps in my back and neck. To the point that condoms have become known as "pain killers" between us.

In my experience, the pain does not return immediately after orgasm, though it certainly doesn't take very long. The endorphins stay in the body for a little while following the arousal and (hopefully) subsequent orgasm.

I hate to be a kill-joy but I have something to add to r00k123's write up. While orgasm usually does bring relief from menstrual cramps it isn't always the case. Personally there have been a few occasions where the strength of uterine cramping has been greatly increased by orgasm and I have been reduced to curling up in agony - something I was not grateful for, in spite of the wonderful feelings of warmth and comfort that can only come from making love.

The increase in strength of the muscular contraction of the uterus (resulting in pain) might be due to prostaglandins present in semen so perhaps an orgasm without intercourse (or without ejaculation into the vagina) would be the answer. More research is needed!

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dwardu kindly informs me that it is the female muscle spasms that accompany orgasm that will cause an increase in frequency and intensity of uterine and cervical contraction and the effect of the prostaglandins would not be felt until some time afterwards.

Masturbation: The Magic Cure-all

Despite the name of this node, what I'm about to say is true.

A few weeks ago I got food poisoning from a fast-food restaurant that shall remain nameless. I woke up with this horrible stabbing pain in my abdomen and spent most of the morning lying on the bathroom floor moaning in pain. When I finally managed to crawl to my couch, Showtime decided they were going to have Drew Barrymore day. So I sat there all day in agony watching Home Fries and Never Been Kissed. Sometime in the afternoon I got really horny. Normally when I'm sick, sex isn't the first thing on my mind anymore. I couldn't shake it I had to masturbate. So I did the deed and in the process my atrocious monster pains ceased. It was amazing; it was better than any pain killer on the market. Maybe it's the serotonin or endorphins. I don't know, but it worked. So every time I felt overwhelmed by pain, I'd have a little fun with myself. That's how I got through the day.

For all the doubters out there, I was talking to Psydereal last night about this subject. One time she so sick and couldn't breathe. Every time she would masturbate she could breathe.

Maybe the medical profession should look into masturbation as a new treatment. Who knows? It can't hurt.