GEOFF BENNETT: The birth control pill is one of the most common forms of contraception in the U.S., but in recent years claims of side effects have filled social media platforms, often fueled by influencers promoting misinformation.

Last night, we brought you a story about some of that misinformation and how contraception is being targeted as part of a larger anti-abortion movement.

Tonight, we take a step back to delve into the science behind birth control.

Special correspondent Sarah Varney reports for our series The Next Frontier.

SARAH VARNEY: Hormonal contraception has been around for decades.

In 1960, the FDA first approved the birth control pill and it quickly became a revolutionary invention.

WOMAN: I want the freedom to choose my own lifestyle.

(CHEERING) ELAINE TYLER MAY, Author, "America and the Pill": This coincided with the feminist movement, and so very quickly the oral contraceptive became associated with the ability of women to control their own fertility.

SARAH VARNEY: Since the original pill, other types of contraception have been developed, from implants to shots to intrauterine devices, or IUDs, all of which are safe and effective.

But in recent years, with the rise of social media, more and more influencers are posting content that questions their safety.

WOMAN: Here are 14 ways that the contraceptive pill robs us of our health.

SARAH VARNEY: Some people have focused their videos on claims that birth control has affected their mental health.

WOMAN: So the side effects while I was on it were completely terrible, depression, anxiety, panic attacks.

SARAH VARNEY: While others say it's unnatural.

WOMAN: Birth control actually depletes your gut of essential vitamins, minerals and your good gut bacteria.

SARAH VARNEY: There are even videos with unfounded claims that it alters romantic and sexual attraction.

WOMAN: Younger age groups of women, like, they seem to go for girlier, more feminine men.

That is -- there's a reason.

Birth control does that to your brain.

SARAH VARNEY: But these claims are not rooted in science, say experts and scholars.

EVELYN KIELTYKA, Maine Family Planning: You hear these wild things.

Birth control methods are one of the most researched hormones in our health care system.

SARAH VARNEY: So let's take a step back and start with how contraception actually works.

In the U.S., about 25 percent of women between the ages of 15 and 44 who use contraception take the birth control pill.

The pill is available in different combinations of hormones, but they work the same way, by preventing a woman's ovaries from releasing an egg each month.

The pill also thickens her cervical mucus, essentially creating a barrier so that sperm are blocked from entering the uterus.

The hormones in the pill can also thin the uterine lining, making it less likely that a fertilized egg will implant, which is when pregnancy begins.

Accounting for missed or forgotten pills, the birth control pill is about 93 percent effective.

Then, there are IUDs, T-shaped devices inserted in the uterus that prevent fertilization.

The hormonal IUD thickens mucus found in the cervix, and the copper IUD disrupts the sperm's movement.

IUDs are one of the most effective forms of pregnancy prevention, about 99 percent.

They can remain in place for three to 10 years, and like all hormonal birth control, once removed, do not affect a woman's future fertility.

EVELYN KIELTYKA: You will be able to get pregnant.

All methods, when you stop a method, you will return to your fertility just like you have before you started that method.

SARAH VARNEY: Some of the women posting online say they're frustrated that doctors don't believe their complaints, while others share examples of bad reactions they have experienced while on birth control, many of which are exceedingly rare.

WOMAN: Story time on how my birth control almost caused me to lose my eyesight.

That's right, my eyesight.

EMILY PFENDER, University of Pennsylvania: What we find is that personal experiences, crying, talking about a really difficult experience, receive significantly higher engagement on social media in comparison to videos and creators that are not doing those things.

SARAH VARNEY: Mild side effects from birth control do happen, from nausea to bleeding between periods, but they ultimately vary person to person.

And clinicians say it's crucial to speak with your provider about what will work best for you.

EVELYN KIELTYKA: It's a very individualized personal choice.

A lot of times, folks say, I heard this or I heard that.

And I really said, well, let's talk about you, because you're the one taking it.

Your body's unique.

We're all -- and how you might respond to a contraceptive method could be totally different from, say, someone who's told you a story.

SARAH VARNEY: Experts also say there is no definitive evidence that any form of birth control causes depression.

And only one method, a hormonal shot, has been found to cause minor weight gain.

Choosing to stop birth control does not lead to infertility.

But many videos highlight possible side effects with little to no context.

EMILY PFENDER: They pull out the FDA insert from their birth control prescription and jokingly talk about how it's so big that they could use it as a blanket to take a nap.

Very, very rare side effects are being talked about as common side effects.

SARAH VARNEY: The rise of influencers giving advice about hormonal birth control concerns those tracking this content.

CHRIS CHOGLUECK, New Mexico Tech: People are making money off of spreading this information as well, especially.

When these individuals have no medical sort of expertise, when they don't have scientific credentials, it's especially worrisome that they're able to build up any sort of career or any sort of like professional clout, because these individuals very likely shouldn't be trusted.

WOMAN: My new budget-friendly program, the 45-day hormone reset, is now live.

To learn more, you can go here.

SARAH VARNEY: In some cases, creators even preface that they are not anti-birth control, but still broadcast their experiences to hundreds of thousands of followers.

WOMAN: I know it actually works and can help you go from pain to power in just three cycles.

SARAH VARNEY: But misleading claims about health effects aren't the only source of misinformation on hormonal birth control.

(CHANTING) SARAH VARNEY: Over the years, conservative Christian and anti-abortion groups have targeted birth control pills, implants, patches, shots, and Plan B.

That's an emergency contraception that contains a hormone that delays or stops ovulation.

It's most effective at preventing pregnancy up to 72 hours after a missed birth control pill, a condom mishap, or unprotected sex.

One formulation can work up to five days to prevent pregnancy.

Despite that medical consensus, activists have flooded social media, falsely equating emergency contraception with abortion.

WOMAN: It can prevent the implantation of a new human being into the uterine lining, because it causes bleeding.

SARAH VARNEY: This argument stems from outdated labeling on the Plan B packaging, which said that the pill might prevent implantation of a fertilized egg.

But, in reality: EVELYN KIELTYKA: By suppressing ovulation, there's simply no egg to be implanted.

It's been well-studied.

If you already are pregnant and you take emergency contraception, it will not dislodge that implantation.

You are pregnant, hard stop.

No amount of E.C.

is going to disrupt that.

SARAH VARNEY: Experts who monitor contraception misinformation worry the endless TikTok's and Instagram posts are convincing people to avoid it altogether.

And they're concerned it will stifle scientific research aimed at improving birth control for everyone.

WOMAN: All the birth control is poison.

Why are they so quick to want to put you on birth control, but they don't even teach you where your womb is?

CHRIS CHOGLUECK: I think the existence of these side effects isn't evidence that these contraceptives aren't safe, but they might be evidence that we want to develop even safer means of contraception for everybody.

SARAH VARNEY: For the "PBS News Hour," I'm Sarah Varney.