“You can’t have a sensible conversation about testosterone,” says Bradley Anawalt, chief of medicine at the University of Washington Medical Center. “Everybody thinks it’s going to make you stronger, faster, better looking, and better at sex. That’s magical thinking. It doesn't do that.” Here’s what “T-therapy” can do, how risky it is, and who might need it. 

What lowers testosterone levels? 

Genetics, chemotherapy, radiation, and injuries to the testicles can all result in a drop in levels of the sex hormone. Some of the more common causes: 

Age

Testosterone levels peak in early adulthood and then begin to gradually decline. “There is, on average, a decline in the testicles’ ability to make testosterone as you age,” says Anawalt. 

“At about age 65 or 70, you see a significant increase in the number of men that have low testosterone levels,” notes Anawalt. Though estimates vary, 10 to 15 percent of men above age 65 likely have testosterone levels that fall below 250 nanograms per deciliter of blood. (Normal blood testosterone levels for men typically fall between 300 and 1,000 ng/dL.) 

Body weight

Excess body fat can also drive down testosterone levels. Why? For one, more body fat means less of the protein that carries testosterone through the blood. But excess body fat also blunts the signals to produce testosterone that are sent from the brain to the testicles. 

In one analysis of 13 studies of roughly 4,300 men with obesity, 43 percent had blood levels of testosterone under 300 ng/dL.  

Losing weight can help. In an analysis of 10 studies, losing roughly 10 percent of body weight through diet changes was linked to a roughly 80 ng/dL rise in testosterone levels.  

Other factors

“Lower serum testosterone levels are commonly encountered in men who are taking opioids or are not getting adequate amounts of sleep,” says Michael Irwig, an endocrinologist and associate professor of medicine at Harvard Medical School.  

If you think about how humans evolved, that makes sense. “If you’re not healthy and need to focus on surviving, your body responds as if it knows that you’re not going to be healthy enough to be a parent,” posits Anawalt. “So the system turns off testosterone and sperm production.” 

“However, low T levels do not necessarily mean that T therapy is the best approach,” Irwig points out. “If I have a sedentary man with type 2 diabetes, high blood pressure, obesity, and sleep apnea, exercise and weight loss will help with all these conditions, whereas T therapy will not.” 

Justin Dubin, a urologist at Memorial Healthcare System in South Florida, agrees. “Having sexual health issues can be a sign of other health problems. Diabetes, obesity, or high blood pressure can be risk factors for erectile dysfunction and potentially low testosterone. I like to tell my patients that what’s good for your heart is good for your parts.”

What are the benefits of taking testosterone? 

Over the past decade or so, four landmark trials looking at a total of roughly 7,300 men have tested the benefits and risks of testosterone replacement therapy for at least a year in middle-aged and older men. (Researchers have largely studied the risks and benefits of testosterone in men with low blood testosterone levels and signs and symptoms of testosterone deficiency. That’s a small percentage of men. So the results don’t apply to men with normal testosterone levels.) 

Here are some of their major findings: 

Sexual function

“The Testosterone Trials looked at older men who had low testosterone levels and had symptoms of testosterone deficiency,” says Anawalt. “Giving these guys testosterone increased their sex drive and sexual desire. And it increased erectile function about half as effectively as Viagra does.” 

Mood and cognition

In the studies that enrolled men who had low testosterone, “there was a positive effect of testosterone therapy on mood and vitality,” says Anawalt. “They felt better, like they had more energy.” But in the studies that enrolled men who had testosterone on the lower end of the normal range, taking testosterone didn’t make a difference. No studies found that testosterone improved cognition. 

Physical function and strength

In the Testosterone Trials, researchers measured how far all the volunteers could walk in six minutes. “This is a test that correlates with longevity,” explains Anawalt. About 20 percent of the testosterone takers—versus only 13 percent of the placebo takers—increased their 6-minute walk distance by at least 164 feet (50 meters). 

“But in the guys that were frailest and couldn’t walk very far, testosterone therapy didn’t improve how far they walked,” says Anawalt. “So if you’re really frail, testosterone therapy isn’t going to do much. But if you’re older and not so frail, it actually might do something.” 

Bottom line: “The effects of testosterone replacement therapy on physical function are probably pretty modest,” says Anawalt. “It’s not going to make you leap tall buildings in a single bound. It’s more like those fancy Nike shoes that increase your running speed by 1 to 3 percent. A slow person is still slow, but it does increase their speed.” 

That said, none of the recent landmark trials instructed the men to exercise. And the trials used only moderate doses of testosterone because the goal was simply to bring levels up to the normal range.  

“If you use a high dose, you get an effect on muscle and strength,” says Anawalt. “But then you’re no longer using testosterone as a hormone replacement therapy. You’re using it as a drug. And when you do that, there’s no question that testosterone increases muscle mass and strength. That’s why athletes use it.” But there are risks to taking testosterone, either as replacement therapy or as a drug. 

What are the risks of taking testosterone? 

Fertility

“One of the biggest risks that most men don’t understand is that testosterone therapy causes infertility,” explains Dubin. That’s because when your brain notes that you have enough testosterone in the blood, it turns off the signal sent to your testicles to make more. Because testosterone in the testicles is essential for making sperm, sperm production plummets. 

And while the effect isn’t permanent, studies using testosterone as a male contraceptive suggest that it can take 6 months to two years for sperm to return to normal after discontinuing testosterone therapy. “The longer you’re on it, the longer it takes to rebound,” says Dubin. 

“So if a patient is interested in future fertility, I will not offer them testosterone replacement therapy. I will offer them alternative medications that can boost testosterone and preserve fertility like clomiphene or human chorionic gonadotropin, also called hCG.” 

Red blood cell count

“Taking testosterone can cause proliferation of your red blood cells,” says Dubin. That may increase the risk of blood clots, though the data are mixed. To monitor the risk, “we check their blood every six months.” Using non-injectable forms (like gels or oral testosterone) or reducing the dosage can lower the risk of excess red blood cells. 

Cardiovascular disease

For years, whether testosterone therapy raised the risk of cardiovascular disease was an open question. At the behest of the Food and Drug Administration, pharmaceutical companies that sell testosterone tried to find out. The researchers enrolled roughly 5,200 middle-aged and older men with hypogonadism (which includes low testosterone levels and symptoms like low libido, fatigue, fewer erections, or loss of body hair) who either had cardiovascular disease or a high risk of it. The men were randomly assigned to use a testosterone gel or a placebo gel for an average of nearly two years and were followed for roughly one more year.

“The major findings are that in men who are treated with low to modest doses of testosterone therapy, there are no observed increases in any major cardiovascular events,” says Anawalt. “That’s strokes, heart attacks, or deaths from cardiovascular disease.”

However, the study did detect adverse events—like irregular heart rhythm, acute kidney disease, or clots in the blood vessels that feed the lungs—that were rare, but more common in testosterone than placebo takers.

While the trial doesn’t completely close the case on testosterone’s effects on cardiovascular risk, it is by far the largest study to look and the only study designed to evaluate that risk. Its results provide some optimism for men with hypogonadism who may have stayed away from testosterone replacement therapy because they were afraid of what it might do to their heart. “You cannot extrapolate from this study to 20-year safety,” Anawalt acknowledges. “But it looks like if there is an effect, it’s unlikely to be large.” 

Prostate cancer

“There’s no signal that testosterone therapy increases the risk of prostate cancer,” says Anawalt. But there’s a critical caveat. 

None of the studies that have tested testosterone replacement therapy lasted more than a few years. “That’s too short to establish whether there is any effect on the risk of prostate cancer,” Anawalt notes. “That’s a disease that develops over many years.” 

“In fact, if you did see something occur in the first year or two, it would most likely be unmasking an underlying prostate cancer, as opposed to causing it. That is, you throw a little kerosene on the cancer, and it kicks off.” 

“The one thing that’s pretty clear is that when you put somebody on testosterone replacement therapy, what you raise is the risk of prostate biopsies.” That’s because PSA, the marker doctors use to alert them to a potential prostate cancer, goes up in most men who take testosterone. “But we think it rises to whatever level they would have had if they had normal testosterone to begin with,” notes Anawalt. 

The bottom line: “We can’t say, one way or the other, whether testosterone replacement therapy increases the risk of prostate cancer,” says Anawalt. 

It’s important to keep in mind that the goal of testosterone replacement therapy is simply to replace testosterone, not to boost it above average levels. Most men with normal levels who take an average dose of testosterone won’t notice much of an effect, Anawalt says. And whether taking additional testosterone when you already have normal levels increases your risk of heart disease, prostate cancer, or anything else is unknown.

How is testosterone deficiency diagnosed? 

“We have men come in all the time as a result of commercials or ads online asking, ‘Do you feel tired? Do you not feel quite so alive? Maybe you should ask your doctor if you have low T,’” says Anawalt. “That’s thanks to a very successful advertising campaign put together by pharma in the early 2000s.” (In response, prescriptions for testosterone ballooned 10-fold in the U.S. between 2000 and 2011.) 

But those and other “non-specific symptoms” like depressed mood and erectile dysfunction could be a result of a host of other problems. “They could be due to inadequate sleep, sleep apnea, vascular disease, or depression,” Irwig points out. 

Having either low testosterone levels or the signs and symptoms of low testosterone isn’t enough. You need to have both to land you a diagnosis of hypogonadism (which is sometimes called low testosterone, testosterone deficiency, or androgen deficiency). 

“Low libido is the most sensitive symptom for detecting low testosterone,” says Dubin. 

Anawalt echoes that. “When we have a man come into our office and tell us that, say, two years ago they used to be very interested in sex and there’s been a sharp decline, that’s a clue that they may have a decline in testosterone.” 

“It’s a decline in the not-so-distant past,” he clarifies. “It’s not, ‘Well, my sex drive is not what it was when I was 18.’” 

Infertility, osteoporosis, and breast growth or breast tenderness can also trigger an evaluation for testosterone deficiency.

“Per the guidelines, low testosterone is less than 300 ng/dL,” says Dubin. It’s important to measure testosterone first thing in the morning, when levels are the highest. And levels need to be measured on more than one occasion because they fluctuate wildly.  

“My testosterone levels could be up to 30 percent lower in the late afternoon than they are in the morning,” Anawalt explains. And about 30 percent of men who have low testosterone according to one measurement will have normal testosterone on the next. 

There’s also variation in how individual men feel at a given testosterone level. Some men may have levels below 300 ng/dL and no symptoms, so screening all men for low testosterone is a bad idea, says the Endocrine Society’s guidelines. 

On the other hand, “if someone comes in and they have a testosterone of 310 and they have symptoms and feel miserable, it’s not fair to say that they don’t have low testosterone,” says Dubin. “There’s an art to medicine, and there is some reasonable wiggle room in how we interpret the guidelines.” 

Can supplements help? 

There’s no shortage of supplements promising to “naturally boost your testosterone,” “blowtorch body fat faster,” or “supercharge your drive and sexual performance.” They typically include ingredients like ashwagandha, fenugreek, Tribulus terrestris, horny goat weed, zinc, and/or B vitamins.  

Spoiler Alert: “There is no evidence that any combination of these or other ingredients in dietary supplements is effective for any claimed benefit you might see advertised to increase testosterone levels,” as the Defense Department’s Operation Supplement Safety website bluntly puts it.  

(Note: As of February 4, 2025, the Operation Supplement Safety website is down due to a cybersecurity issue, according to a spokesperson for the organization. We’ve linked to a pdf of the previously published version.)

While many studies (often company funded) suggest that some of those ingredients can boost testosterone, the results don’t hold up to scrutiny. 

“Those supplement studies are a bunch of hooey,” says Anawalt. “They’re fundamentally flawed.” 

Why? “If you want to make something look good, you can,” he says. And that’s particularly easy to do with testosterone, in large part because blood levels vary so much throughout the day and between days. “If you do only one blood sample at each of these time points, just by chance you could see an effect,” Anawalt says. Good luck finding a supplement study that measured testosterone more than once at the beginning and once at the end of the study. 

Then there’s time of day. “Testosterone is highest between 7 and 10 in the morning,” notes Anawalt. If you measure later in the day at the beginning of the study and earlier in the day at the end of the study, odds are high that you’ll show a “boost” in testosterone.  

What’s more, testosterone is notoriously tricky to measure. “There are many bad tests out there,” says Anawalt, who adds that there is a national effort to standardize testosterone tests. “Many are now certified by the Centers for Disease Control and Prevention.” 

Most studies don’t provide enough detail to know exactly when testosterone measurements were taken or how testosterone was measured.  

But a bad test was a problem with a 2019 ashwagandha study. The company-funded randomized trial reported that testosterone levels in middle-aged and older men were roughly 15 percent higher in ashwagandha takers than in placebo takers after 8 weeks. But the researchers measured testosterone levels in the men’s saliva, which isn’t a method validated by the CDC. 

Even if you put all those issues aside, many trials still find no effect of supplements on testosterone levels. In one review article, only 9 of 32 trials that tested various herbs and botanicals found a benefit. And in most of those cases, the results were ho-hum. Most studies were conducted in men with normal testosterone levels who ended up with marginally higher—but still normal—levels. Don’t expect a 15-or-so percent change in already-normal testosterone levels to “send your libido into hyperdrive” or “pack on slabs of lean muscle,” as one supplement’s website claims. 

A changing marketplace 

Testosterone therapy is a booming business. “This is the hottest topic for men,” says Dubin. And that’s no longer true just for middle-aged and older men. In one not-yet-published survey, Dubin found that 40 percent of men under 40 were interested in taking testosterone and 14 percent had used testosterone therapy or taken a testosterone booster (like anabolic steroids). 

And ads for testosterone therapy have expanded from TV to social media and online advertising, points out Irwig. The barrage of ads and easy access to doctors who prescribe testosterone has convinced many men that they need to take the hormone. 

Online and for-profit men’s health clinics 

You no longer need to visit your doctor’s office to get your hands on a testosterone prescription. “There’s been a proliferation of online companies and for-profit men’s health clinics,” says Irwig, who has written about the off-label use of testosterone. (Off-label means that a healthcare provider is prescribing a drug for a purpose or in a population that has not been approved by the FDA. In the case of testosterone, that often means that it’s prescribed for men with normal T levels. The practice is perfectly legal.) 

Irwig is talking about “direct-to-consumer” online health platforms that offer telehealth services and brick-and-mortar “Low-T” clinics that provide testosterone and other medications specifically for men’s health. Many don’t accept insurance. Instead, they make money offering testosterone prescriptions (and medications for things like hair loss or erectile dysfunction) directly to men, often included in pricey monthly membership fees. 

“Achieve your peak testosterone levels,” says the website of the direct-to-consumer company Maximus, which offers testosterone replacement therapy for roughly $100 to $150 a month. “Boost energy, build muscle mass, and feel like your younger self.” 

“Many of these companies try to get men to think that all of their bothersome symptoms—low energy, lower libido, erectile dysfunction, weight gain—are due to low testosterone, and that testosterone therapy will suddenly make their symptoms disappear,” says Irwig. “But many of these companies prescribe testosterone to men with above-average levels and mislead customers by telling them that their normal T levels are low.” 

Dubin has had to deal with some of the fallout from that practice in his clinic. “We were seeing a spike in people who were coming in on testosterone who didn't know that it caused fertility issues, and they were getting it from direct-to-consumer companies,” he says. 

So he decided to try out direct-to-consumer care on himself. “I pretended to be someone who had symptoms of low testosterone like low energy and libido who was interested in future fertility,” he says. “Before I started, I got blood work. My testosterone was 675 ng/dL, so I was clearly not a good candidate for testosterone replacement therapy.” 

Dubin met with telemedicine providers from seven direct-to-consumer companies. “Six out of the seven offered me testosterone replacement therapy, and I was not informed about infertility by half of them.” 

Half of the platforms that offered him testosterone said that their goal was to get his testosterone levels up to or above 1,000 ng/dL. While there are no hard and fast rules about what testosterone levels should be, they should not be above 1,000 ng/dL, says Dubin. “When we treat men with testosterone replacement therapy, our testosterone level goal is 450 to 600. We want to keep you below 1,000.” 

The role of direct-to-consumer care 

Many men may feel embarrassed about seeking care to treat men’s health issues. Or they may not live in an area with easy access to care. So direct-to-consumer telemedicine services, and even for-profit men’s health clinics, may feel more approachable, less stigmatizing, or more convenient, Dubin points out. 

He encourages men who have used—or are considering—online services or for-profit men’s health clinics to ask themselves a few questions. 

1. “Who is this person that I’m trusting my health to?” In one study of roughly 220 direct-to-consumer clinics specializing in men’s health, nearly 90 percent did not list an endocrinologist or urologist on staff on their websites. But many listed naturopaths or chiropractors. Others had no physician on staff. 
2. “Have they educated me about the risks and benefits of taking this medicine?” 
3. “Is the cost reasonable?” 
4. “Do I want to see someone else before making a decision on my health? If you’re not comfortable with someone’s plan, you can always explore other options.” 

Irwig provides more food for thought. “The goal of for-profit men’s health clinics and online companies is to make money by prescribing medications such as testosterone. In contrast, as a hospital-employed endocrinologist, I have no financial incentive to prescribe a particular medication. Instead, I strive to base my recommendations on what I think is best for the patient’s overall health.”