AMNA NAWAZ: More than 60 years after a birth control pill was developed for women, there is still no similar federally approved drug for men.

But promising results using a new gel for men might finally change that.

William Brangham has the details.

WILLIAM BRANGHAM: Amna, while this drug is still being tested, it has already generated a lot of interest.

It's a clear gel that you rub on your shoulder once a day.

And early results show that it blocks sperm production after two to three months of daily use.

Dr. Brian Nguyen an associate professor of clinical obstetrics and gynecology at the USC Keck School of Medicine.

And he was involved in this current study.

Dr. Nguyen, thank you so much for being here.

So how does this gel work?

What's the mechanism of action here?

DR. BRIAN T. NGUYEN, Keck School of Medicine of USC: Well, first of all, thank you so much for having me.

I'm so glad to hear that there's more and more interest in male contraception.

In terms of the way that this works, it is a hormonal method.

And so to understand the hormonal system here requires you to understand that it works kind of like a thermostat that's always measuring how hot or cold your room or your house is.

And, in this case, your thermostat is measuring how much testosterone is in your body.

And so, if you give testosterone, it will there's too much, in the same that, if it's too hot, it'll sense that it's too much.

And therefore it'll turn off the heater.

So, in this case, you're turning off the testes.

You're turning off the production of testosterone in the body, which will also turn off the production of sperm.

What's nice about that is that we know that this factory is intact.

All you're doing is you're flipping some switches so that, when you flip the switch back on, then you're going to have reversal of the sperm production and you will have full production once again.

WILLIAM BRANGHAM: So the hormones that are applied bring sperm production down to a low enough level so that a man couldn't impregnate his partner?

DR. BRIAN T. NGUYEN: That's correct.

The majority of men actually get down to zero sperm, but we actually know that you don't need zero sperm to prevent pregnancy.

Actually, our threshold is about one million per milliliter.

WILLIAM BRANGHAM: You can still have a million sperm per milliliter and not get someone pregnant?

DR. BRIAN T. NGUYEN: It sounds a little bit crazy to say, but we -- our threshold for actually a lowest sperm count is about 20 million.

So, when you consider that fact, right, dropping down by an order of magnitude really does reduce your risk of pregnancy.

WILLIAM BRANGHAM: OK.

So, the -- I know you haven't published your full results yet, but how promising does this look for being a legit male contraceptive?

DR. BRIAN T. NGUYEN: Well, I think it's extremely promising.

And I'm so glad you brought that up, because we actually do have trials in the past that have shown us that hormonal methods are capable of preventing pregnancy.

And so we feel very confident about that.

And so as long as we can drop the sperm counts down to where we want them, then we have got a method that should be able to prevent pregnancy.

WILLIAM BRANGHAM: And I know you spoke with some of the participants in this trial.

Did they describe any side effects that you have been documenting?

DR. BRIAN T. NGUYEN: Well, certainly.

It's a hormonal method.

And so we definitely do talk about potential side effects that can be involved, mood changes, weight changes, changes in libido that can be up or down, acne as well.

Our participants do mention that they do experience these changes.

However, the question is, how bothersome are these side effects to them?

And for these men or these couples to stay in the trial for more than a year using exclusively this method on a daily basis really is proof of the pudding that we got something here.

WILLIAM BRANGHAM: Dr. Nguyen, why has it taken so long to get to this point?

I mean, I understand that studies were done initially on this back in the 1970s, but, for all of those decades, this burden has largely fell -- fallen on women.

Why has it taken so long for a male contraceptive?

DR. BRIAN T. NGUYEN: So many reasons, from a pharmaceutical industry standpoint, also from social biases.

But my take of it is that there's also been this thought that men don't want to be involved or don't have enough knowledge about reproductive systems and their responsibility to really engage in this responsibility.

I think things are changing.

And I would say that, in this last year, we have received such a huge uptick in media interest in male contraception that it seems like there's kind of a shift in what I would consider expectations for gender equity.

WILLIAM BRANGHAM: Let's say all of this goes smoothly and the tests work out as you hope they do.

When might we see a product on the market?

DR. BRIAN T. NGUYEN: It's commonly said that anywhere between five to 10 years is about an appropriate timeline.

I would say that, thanks to folks like yourself who are giving us a little bit more time in the spotlight, it might increase the demand.

That increased demand and that public awareness is what is going to drive the speed of how things go for us.

WILLIAM BRANGHAM: All right, Dr. Brian Nguyen at the USC Keck School of Medicine, thank you so much for talking with us.

DR. BRIAN T. NGUYEN: Thank you so much.

Take care.