SAN ANTONIO -

Testosterone replacement therapy has become a $2 billion business over the last five years.

According to Global Industry Analysis, a "worldwide business strategy and intelligence source," the global market will surpass $5.1 billion by 2018. 

The airways are full of ads on TV and radio, and billboards advertising the therapy dot the highways.

It is a fact that testosterone levels in men decrease with time.

“As we age, the testosterone gradually declines, and so that is one of the things we see with aging," said Dr. Ian M. Thompson, director of the Cancer Therapy and Research Center at the UT Health Science Center.

Drug companies and clinics are trying help men replace that loss.

“We know more about the disease now (and) we can recognize the symptoms,” said Dr. Bill Riley, a former orthopedic surgeon who does marketing for the Low T Center.

Those symptoms include fatigue, weight-gain, muscle loss and low sex drive.

The Low T Center have some 25,000 patients.

When they first come into the clinic, they are given paperwork to fill out, then a testosterone test.

If it is deemed they have low testosterone, patients can then be put on a program.

But not everybody qualifies.

“We don’t just treat anybody that walks through the door. We treat maybe 60 percent of the patients,” said Riley, who is 64 and been a patient for two years.

Low T gives their patients shots of testosterone once a week.

"You come in every week for a shot (and) we monitor exactly how much you get,” Riley said.

They also do complete lab work every three weeks.

“What the evidence suggests today is that there are some potential side effects,” Thompson said. "The trick is to make sure that the patient that’s starting (the therapy) knows that there is not just benefits, but there are potential risks."

Those risks include heart disease, higher blood count and higher prostate-specific antigens.

There have been several studies over the years related to testosterone therapy. The latest is a VA study that showed patients on the therapy experiencing cardiac events.

“I question the quality of the study (and) the methods of how it was performed, " Riley said.

Both doctors agree more studies need to be conducted.

Thompson said he would like to see a study with anywhere from 60,000-100,000 patients.

Riley says the Low T Center is using their patients to begin a research program.

Thompson did have a piece of advice for men considering testosterone replacement therapy: “The informed patient knows the benefits, knows the potential risks and can make an informed decision."

Thompson said there is also a less risky but much harder way to get testosterone levels higher -- lose weight.