Young women on birth control pills tend to have less painful menstrual periods than those not on the contraceptives, a new study finds.

Swedish researchers found that of 2,100 women followed from age 19 to 24, those on the combined birth control pill (estrogen and progestin) had less-severe menstrual pain over time.

It's already common practice for doctors to recommend the Pill to women with dysmenorrhea -- menstrual cramps, back pain and other symptoms that are severe enough to disrupt a woman's life.

Birth control pills are not specifically approved for that purpose, but doctors can prescribe them for dysmenorrhea on an "off-label" basis. However, it has not been clear how effective the pills are against period pain.

The new findings are not conclusive, but still caused excitement among some researchers.

"Our study provides evidence for the effective relief of painful periods with combined oral contraceptives," said Dr. Ingela Lindh of Gothenburg University in Sweden, who led the study.

Both Lindh and one of her co-researchers have financial ties to companies that make hormonal contraceptives, although the new research was not supported by drugmakers.

Menstrual pains typically fade as a woman gets older, and they often lessen after childbirth. But even when age and childbirth were taken into account, Pill users had less painful periods in the new study, Lindh told Reuters Health in an email.

The study, published in the journal Human Reproduction, does not prove that the Pill eases dysmenorrhea.

It's an observational study that looked at the relationship between women's Pill use and dysmenorrhea risk. Clinical trials -- where people are randomly assigned to take a drug or a placebo -- are considered the "gold standard" for proving cause-and-effect.

And a 2009 review of 10 clinical trials concluded that there was "limited evidence" that the Pill improved menstrual pain.

Still, the trials in that review varied in their methods and their quality, so it's hard to draw firm conclusions, according to Dr. Michele Curtis of the University of Texas Medical School at Houston, who was not involved in the current study.

She told Reuters Health the new study isn't definite, but "makes a strong case" that the Pill is effective against menstrual pain.

"I think combined oral contraceptives really do help women with primary dysmenorrhea," said Curtis, who has received speaking fees from drugmaker GlaxoSmithKline, according to ProPublica's database Dollars for Docs.

Primary dysmenorrhea refers to menstrual pain that is not caused by underlying medical conditions such as endometriosis, a disorder of the uterine lining, or non-cancerous uterine growths called fibroids. When a medical condition is the cause, it's known as secondary dysmenorrhea.

A weakness of the current study, Curtis said, is that it did not determine whether women had primary or secondary dysmenorrhea. In some cases of secondary dysmenorrhea, she said, birth control pills might help, but in other cases will do nothing.

The study included three groups of young women who were 19 years old in either 1981, 1991 or 2001. They all completed a standard questionnaire on menstrual symptoms, then repeated the survey five years later.

Dysmenorrhea was common, the study found. Of the 1981 group, 37 percent had at least moderate menstrual pain that disrupted their daily activities; in the 2001 group, that figure was 47 percent.

But Pill users had less pain over the next five years. Overall, Pill use was linked to a reduction of 0.3 units on the pain scale. That means every third woman on the Pill went "one step down" on the scale -- from severe pain to moderate pain, for example -- according to Lindh.

The researchers also looked at subgroups of women who were using the Pill at the age of 19, but not at age 24. On average, their menstrual pain increased over time. In contrast, pain decreased among women who were not on the Pill at age 19, but were at age 24.

There are biological reasons that the Pill would help with dysmenorrhea, both Lindh and Curtis said.

Menstruation causes increased muscle activity in the uterus, which lessens blood flow to the uterus. And that's believed to be the root of menstrual pain.

Hormone-like compounds called prostaglandins help churn up that extra muscle activity. Since birth control pills lower the body's prostaglandin production, Lindh explained, it makes sense that they would ease dysmenorrhea.

Birth control pills, which cost anywhere from $15 to $50 a month, are not the only treatment for dysmenorrhea.

Some women can find enough relief from nonsteroidal anti-inflammatory drugs (NSAIDs), like ibuprofen and naproxen, according to Curtis. NSAIDs also block prostaglandin production.

What's more, vitamin B1 and magnesium, exercise, relaxation techniques and acupuncture have all been advocated for dysmenorrhea.

"Clearly, our evidence base is smaller for those things," Curtis said. But she also said that if a woman does not want birth control or NSAIDs, she could try an alternative.

Birth control pills can have side effects like breast tenderness, nausea and vomiting, and spotting between periods. Pill users also have a slightly higher-than-average risk of blood clots, particularly if they smoke or are age 35 or older.

But most women with primary dysmenorrhea are younger. In fact, Curtis said, if you start having painful periods for the first time when you are 30, it's unlikely that it's primary dysmenorrhea. A secondary cause is probably at work.

The new study was funded by grants from the Gothenburg Medical Society and other groups.