Guys, this one’s for you. If you’re tired all the time and just don’t feel like getting it on any more (unlike in the past), you’re not alone—but what gives? It could have to do with testosterone. The hormone plays a huge role in men’s libido and overall health, but it declines with age.

What Is Testosterone—And Why Is It So Important?

Testosterone is the primary sex hormone in men. That means it plays a key role in developing muscle, growing hair, regulating mood, and fueling your sex drive, says Christopher Asandra, M.D., chief medical officer of NuMale and NuFemme, medical centers that specialize in sexual health and treating sexual dysfunction.

Basically, testosterone is what makes men ‘men,’ at least from a physical standpoint—but that’s not all. “Testosterone also influences emotion and behavior—not just in the developing male, but the adult and aging male as well,” says John Robinson, a board-certified naturopathic medical doctor and founder of The Hormone Zone in Scottsdale, Arizona.

We tend to think of testosterone as what makes guys act like jerks, says Robinson, as it plays a role in traits like assertiveness, risk-taking, and competitiveness. But it’s not all so stereotypical—recent research also links testosterone with other social behaviors and traits like confidence.

What Happens To Testosterone As You Age?

Once you hit 30, your T-levels start to decline by about one to two percent annually, says Asandra.

After hitting the big 3-0, you may start to experience fatigue, decreased mental clarity, irritability, depression, and decreased muscle mass. “I hear that [last] one a lot,” says Asandra, who notes that a drop in testosterone levels could be to blame for you not being able to lose fat or put on muscle—even if you hit the gym regularly.

Related: 8 Possible Reasons Why You’re So Tired All The Time

And, of course, your appetite for sex might start to wane, too: “The frequency and intensity of erections may start to go down,” says Robinson.

What About Testosterone Boosters?

Chances are, you’ve heard about supplements that can help boost your testosterone—but what’s in them, and should you start taking them?

One of the most common ingredients found in testosterone boosters is fenugreek extract. A 2010 study published in the Journal of the International Society of Sports Nutrition found that taking 500 milligrams of fenugreek extract in supplement form had a positive impact on upper and lower-body strength compared to a placebo. (Remember: When your T drops, you may face a loss of muscle mass.)

Two other popular ingredients—vitamin B6 and vitamin B12—have been the subject of multiple animal studies. A review out of University College London suggests that B6 plays a role in mediating the action of steroid hormones like testosterone in the body, which may explain why you’ll see it on many supplement ingredient lists.

If you are considering taking a testosterone booster, it’s important to see your doc first, says Asandra. You could be dealing with another medical issue, like diabetes, which may be mimicking the signs of low testosterone. And since some people may need a considerable dose to see an effect, you should also check with your doctor if you take other medications, like blood thinners or antidepressants, before adding it to your supplement regimen.

How Can You Replace Testosterone?

Robinson says boosters tend to work in younger guys, but that once you hit 50 and start to experience an exponential decrease in the hormone, you may need to talk to your doctor about hormone replacement therapy.

Yep, there are ways your doc can help you actually replace lost testosterone. Both Asandra and Robinson like to use pellets that are placed under the skin and pump out hormones slowly over the course of four to six months. Testosterone injections are another option—but keep in mind that you’ll have to stick yourself with a needle once or twice a week.