What’s so important about testosterone? Plenty. This hormone plays a part in the development of the penis and testicles, deepens a boy’s voice when he hits puberty, and strengthens muscles and bones. It also gets a guy interested in sex and helps his body produce a steady supply of sperm. Scientists think it even influences mood.

Testosterone production ramps up at puberty, reaching its maximum around age 17. After that, levels remain high for the next 2 to 3 decades before slowly starting to dip. For some men testosterone levels drop off sooner than they’d like.

The American Urological Association estimates that about 2 in every 100 men — and 1 in 100 young men — have low testosterone. This is sometimes called Testosterone Deficiency Syndrome (TD) or simply Low T. And the older you get, the more likely you are to have it. In fact, as many as half of men over age 80 are affected. When your testosterone levels drop, you might start noticing these symptoms:

  • Less body and facial hair
  • Loss of muscle mass
  • Low sex drive or impotence
  • Irritability, lack of concentration, depression
  • Fatigue

But there’s good news: There’s a simple, effective way to treat low testosterone. “If you’re experiencing symptoms such as fatigue and sexual problems, your doctor can test you for your testosterone levels,” says Martin Gross, MD, a urologist specializing in men’s health at Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire. “If they’re low, supplementing can really help.” And one of the most effective forms of testosterone supplementation, Dr. Gross says, is testosterone cypionate.

Have you received a prescription for testosterone cypionate? We spoke with Dr. Gross to find out everything you need to know to get the most out of your treatment.

How does testosterone cypionate work?

Dr. Gross: It’s an injectable medication, a shot in your butt every 2 weeks. It gives you a hefty increase in testosterone levels very quickly. So for the first 5 or 6 days, your testosterone levels will be higher than usual. The next 5 days, your levels will be just about average. Then for the last 3 or 4 days, they’ll start to taper off to your baseline.

For about 10 days out of the 2-week cycle you’ll feel great. Even when your levels start to fall off, they’ll still be above your average before you started. And then you’ll be ready to start your next cycle.

What if those swings during the 14-day cycles bother me?

Dr. Gross: Some men may be bothered by the swings that occur between one shot and the next, and some really mind it when they come back down after feeling great. But we have ways to mitigate those fluctuations.

We can do a 7-day cycle, which may work better for some people. That way, you’d have 3 days when levels are up, 3 days when they’re average, and 1 day when they’re tapering down. We can even do a quarter-strength shot twice a week. The schedule period is very modifiable.

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How do I get the testosterone cypionate shot?

Dr. Gross: When you visit your health care provider, someone on the care team will give you your first injection and show you how to administer the shot to yourself at home. You get it in your butt, and for most guys, it’s very easy for them or their partner to do.

Are there any side effects to testosterone cypionate?

Dr. Gross: The first thing you might notice is pain or soreness at the injection site and some increased acne. You might also notice that your testicles, which make testosterone, are reduced in size. When your brain signals that there’s already enough testosterone via the injections, your testicles will make less of the hormone on their own. This causes them to noticeably decrease in volume.

Also, when you’re on a testosterone supplement, your testicles won’t make as much sperm as before, which could be a problem if you’re concerned about fertility. So it’s not a good choice if you and your partner are trying to have children.

Increased aggression is a known issue, too, but it generally occurs only with very high doses, usually when it’s being abused. When you’re taking testosterone under the care of a doctor, that’s less likely to happen. You’ll come to the office every 6 months to monitor your blood levels of testosterone, check your PSA levels to make sure you don’t have prostate cancer, and adjust your dose if needed.

Will testosterone cypionate cause heart problems?

Dr. Gross: That’s a popular misconception that was based on older research that looked at an already unhealthy population. Recent studies indicate that testosterone may actually be good for men’s cardiac health. It can have some protective benefits, while low testosterone may put guys at higher risk for heart disease.

In my practice, among patients on testosterone, there have been no increased heart events. On the other hand, people with serious cardiovascular disease may complain of fatigue and low muscle mass. That’s likely because they have a chronic illness that’s impeding their overall health. They would need to fix that problem first. Giving them testosterone isn’t the solution.

Is there anybody who should definitely not get testosterone cypionate?

Dr. Gross: Men who have prostate cancer, or who are suspected of having it, should not receive a testosterone supplement of any kind. And if they’re already on a testosterone supplement, they should stop right away. That’s because prostate cancer cells rely on a steady stream of testosterone to keep growing.

Low testosterone is just one possible cause of erectile dysfunction. Learn more about the medications that can help you get your mojo back.