The testicles make the hormone testosterone. This hormone helps in the formation of male sexual characteristics and plays a major role in maintaining muscle mass and healthy bone density. Healthy testosterone levels also fuel a man’s sex drive and a positive mental outlook.

However, testosterone production starts to decline starting around age 30. A blood test can determine your testosterone level and whether you fall into a low, high, or normal range. You may want to consider testosterone therapy if your levels drop significantly.

Artificial testosterone is available as an injection, a patch, or a gel. This type of hormone replacement therapy was shown to be generally safe in the past. Recent research suggests that it may have higher cardiovascular risks than were understood previously.

In January 2014, the U.S. Food and Drug Administration (FDA) issued a safety warning about the possible connection between testosterone therapy and increased risk of heart attack or stroke. The FDA didn’t conclude there was a definite risk. Instead, it noted that recent research has raised these concerns. Doctors and patients are encouraged to report side effects of testosterone therapy to the FDA’s MedWatch.

One of the studies the FDA based its warning on was published in PLOS One. Researchers in this study found that some men faced a substantially increased risk of heart attack after starting testosterone replacement therapy. The men most affected were younger men with preexisting heart disease, as well as older men.

The researchers suggested that testosterone may make blood more likely to clot. Clots that block blood flow to the heart or brain can lead to heart attacks or strokes. Based on these findings, researchers urged doctors to explain to patients how testosterone replacement therapy may affect their heart health.

An increased risk of sleep apnea is another aspect of testosterone therapy that affects cardiovascular health. With sleep apnea, you temporarily stop breathing many times while you sleep. Sleep apnea can raise your blood pressure, which your risk of stroke. It’s also associated with higher risk of heart valve disease and dangerous heart rhythms called arrhythmias.

Testosterone therapy may increase your cholesterol levels. Increased cholesterol buildup in the arteries supplying blood to your heart can lead to a heart attack. Other side effects include oily skin, fluid retention, and a decrease in the size of your testicles. Receiving testosterone therapy may also affect your natural production of testosterone if your hormone levels are normal.

Hormone replacement is associated with certain side effects, but this therapy does help many men restore a diminished sex drive and build muscle mass. As people age, muscle mass tends to decline and your body tends to retain more fat. Testosterone may help reverse those trends. However, if you’re going to take artificial hormones, you should only do so under your doctor’s guidance.

Researchers continue to explore the risks and benefits of testosterone therapy. While testosterone may seem like a fountain of youth for many men, you should be cautious about using it. Have a comprehensive discussion with your doctor about what testosterone replacement therapy can and can’t do. Be sure to look at the possible side effects before you make a decision.