An attempt to develop a safe and effective "male pill" is making headway, according to preliminary results of a small study. In a four-week trial of men under 50, an experimental hormone-based birth control pill was found to be "well-tolerated."

And participants' testosterone levels dropped significantly along with two hormones essential for sperm production, the U.S. study team noted.

Study author Dr. Stephanie Page described the results as "a promising step forward" in the development of a male version of the female birth control pill.

But don't ditch your condoms just yet. Page added that "larger, longer-term studies are clearly needed to address potential side effects."

Interest in a male birth control pill is strong, she said.

"Women have many options, but many women cannot use hormonal and other methods available to them," said Page, who heads the University of Washington's division of metabolism and endocrinology, in Seattle.

"Men are increasingly interested in sharing the burden of contraception, as well as controlling their own fertility," she noted.

For the study, researchers evaluated three doses (100, 200 and 400 milligrams) of a once-daily contraceptive prototype called dimethandrolone undecanoate (DMAU). Two formulations were tried inside the capsules, either powder or castor oil.

DMAU combines activity of a hormone like testosterone and a progestin. It was developed by the U.S. National Institutes of Health, which funded the study.

It differs from prior attempts to create a male birth control pill in several ways. It contains just one steroid, rather than two, and was not associated with any liver toxicity, something that has plagued prior male pill efforts, Page said. And "unlike other oral testosterone derivatives, DMAU only needs to be dosed once a day," she added.

For the study, the researchers randomly divided 100 men into groups of 20 or less. Some men were given a sugar pill (placebo), while others were given a daily oral dose of DMAU at one of the three chosen doses. DMAU was always consumed with food.

Blood tests revealed that at the highest dose, DMAU suppressed production of testosterone and two other hormones -- LH and FSH -- known to be key to sperm production.

But none of the study participants displayed complications that might arise from testosterone deficiency, such as mood changes or disturbances in terms of sexual function.

However, all who took DMAU experienced mild weight gain (roughly 3 to 9 pounds), and a mild drop in so-called "good" cholesterol (HDL). No serious side effects were reported.

Page added, however, that a drop in sperm-producing hormones "is not the same thing" as demonstrating an actual drop in sperm count.

"We need to do a longer trial -- three to six months -- to demonstrate that sperm productions falls with longer use of DMAU," she said. Twenty-eight days "is not long enough" to fully answer that question, she explained.

"However, we know from our work, and those of many others in the field, that the degree of 'sperm supporting' hormone suppression that we observed in this study should be adequate to prevent sperm maturation," Page said.

She reiterated that "this was a very small study, and there is still a lot of work to be done." That follow-up effort is getting underway at the University of Washington and LA Biomed Harbor-UCLA.

According to Page, multinational surveys indicate that a majority of men are interested in more options for reversible contraception.

Currently, the only reversible male contraceptive is the condom, which is not the most reliable method of birth control, she pointed out.

The study results were presented Sunday in Chicago at a meeting of the Endocrine Society. Research released at meetings is generally considered preliminary until peer-reviewed for publication in a medical journal.