In case you haven’t noticed, advertisements aimed at treating low levels of testosterone, or “low T,” have displaced those erectile dysfunction commercials with side-by-side bathtubs. The ads ask: Are you suffering from any of the following — depression, low energy, weight gain, fatigue, low sex drive?

Really, at midlife, who isn’t?

“Those symptoms are true of everybody as they age, to a greater or lesser extent,” says Glenn Braunstein, an endocrinologist and vice president of clinical innovation at Cedars-Sinai Medical Center in Los Angeles. Low T, he says, is the latest trend in direct-to-consumer advertising, promoting such products as AndroGel, Testim and Axiron that deliver the male sex hormone through the skin — a more convenient and less painful option than the injections that have been available for decades.

These drugs, which require a prescription, treat hypogonadism, or low testosterone production. While doctors agree that testosterone therapy is beneficial in hypogonadal men, they are concerned about rejuvenation clinics and Internet sites that push testosterone — or supplements dubiously claimed to boost it — as a cure-all for aging symptoms.

Radio and TV spots suggest that testosterone can cure depression, improve mood and sexual performance, boost energy and melt away pounds. While those symptoms can all be signs of too little testosterone, they are also caused by other conditions, many of which can be treated with changes in diet, exercise and lifestyle. Hormone experts say that using testosterone as a quick fix for aging may be misguided or, worse, unsafe.

More men than ever are asking their doctors about the newer testosterone products. A study published this month showed that between 2001 and 2011, testosterone prescriptions for men age 40 and older more than tripled, with the topical gels being the most popular form and men in their 40s the fastest-growing group of users. The report also found that more than a quarter of men prescribed testosterone had not had their levels of the hormone checked before they were given a prescription.

Replacement testosterone therapy in men 40 years and older who have low levels of natural hormone “is legitimate, when done legitimately,” says John Morley, an endocrinologist at St. Louis University School of Medicine. But, he contends, giving testosterone to men with normal levels of the hormone, especially over the long term, holds unknown risks.

He recently reviewed the scientific evidence behind testosterone and other rejuvenation hormones and found “little evidence” that human growth hormone, dehydroepiandrosterone (DHEA) and melatonin hold any benefit. While testosterone improves sex drive, erectile function, muscle strength and bone density in men with low T, there are very few studies examining benefits or risks in normal men.

But doctors disagree about the waning of testosterone that occurs as men grow older: Is it a disorder to be treated or a natural part of aging? Estimates of how many men in their 60s have low T range from 1 percent up to 70 percent, depending on how doctors assess the condition, whether by symptoms alone or in conjunction with a hormone blood test.

The normal range of testosterone is 300 to 1,000 nanograms per deciliter of blood. After age 30, men typically lose 1 percent of their testosterone per year. Testosterone levels fluctuate throughout the day, with the highest peak in the morning. They can drop due to illness, depression, interactions with other medications and even having a newborn in the house, says Braunstein.

To diagnose low T, a doctor ideally should take two blood samples between 7 and 10 a.m. Low T is generally defined as a testosterone level below 300, accompanied by one or more of the classic symptoms: low sex drive, soft erections, low energy and weight gain. A study last year of almost 3,000 men ages 40 to 79 found that 2 percent had testosterone levels below 320.

“For a drug, testosterone’s relatively safe,” Morley says. “But no studies go longer than three years. What happens if you take it for 20 years?” If large numbers of men begin taking testosterone in their 40s to combat normal aging, he says, “are we going to see similar problems in aging populations that we saw in the Women’s Health Initiative with estrogen [replacement therapy,]” which turned up small but significant increases in cardiovascular and cancer risks? “Probably so.”

And the known risks of testosterone therapy are enough to keep Braunstein and many doctors conservative about its use. About 40 percent of men receiving injections and up to a fifth of those using topical testosterone develop a high blood cell count, a condition known as polycythemia. Thickened blood increases the risk of blood clots, stroke and heart attack.

“The biggest worry is prostate cancer,” Morley says. Both he and Braunstein point out that current data, from small studies, suggest that testosterone therapy probably does not cause prostate cancer, but it may aggravate a prostate cancer already present. Morley says that physicians treating low T should monitor patients closely for both prostate cancer and polycythemia.

Another side effect of testosterone could get overlooked in the doctor’s office: infertility. Taking testosterone can shut down sperm production.

“This is a risk factor patients need to know about,” says Jared Moss, a resident urologist at the University of Tennessee Medical Center in Knoxville, who recently reviewed the effects of rejuvenation hormones on sperm count. “Guys are having children at older ages; doctors must take that into account and discuss the potential for infertility.” The data suggest that sperm counts can recover once testosterone is stopped, Moss says.

Morley regularly treats men in their 60s, 70s and even 80s with a hormone level of lower than 300 who also complain of low sex drive or muscle weakness. “There’s nothing wrong with trying it for three months. But realize you are doing it for your well-being and quality of life, not because you will live longer.”

Braunstein is more cautious: “If you don’t need it, don’t take it,” he advises. “There’s absolutely no evidence that these treatments decrease aging. Ponce de Leon never found the fountain of youth, and the baby boomers won’t, either.”