Testosterone replacement therapies come in many forms, according to Nelson Vergel, author of "Testosterone: A Man's Guide." The hormone can be injected into muscle, absorbed through the skin via a cream/gel, or released slowly through a small pellet that's inserted into the body. A doctor does regular blood tests to determine the correct dosage. 

No magic pill 

You've likely read that there's no magic pill for perfect health. While direct-to-consumer marketing may make it seem otherwise, testosterone is no exception, Vergel says. 

"I'm just amazed how many men start a hormone without doing research," he says. "It's a wonderful thing to start if you need it. It also has some side effects if not done properly." 

The hormone will help you lose weight and build muscle, but not without proper exercise and nutrition. It will also improve your sex drive; what it won't do, Vergel says, is make you into a teenage Casanova.

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