You may have seen a low testosterone (low T) ad on television: A middle-aged man complains of low energy, low sex drive, and irritability. His health care provider gives him testosterone, and he is good to go again. Could that be you? The truth is, not everyone should take testosterone therapy, even with low testosterone. Your testosterone peaks when you are about 17 and starts dropping naturally after age 30. If you are older than age 60, there's about a 20% chance that you have low testosterone. If you're between ages 40 and 60, you have about a 7% chance of low T. Lower testosterone is a normal part of aging. Not everyone has noticeable or bothersome symptoms. However, if you have low testosterone symptoms, you might benefit from testosterone replacement therapy. How to Know If You Have Low Testosterone Your health care provider can do a simple blood test to measure your testosterone level. Testosterone is measured in nanograms per deciliter (ng/dl). Health care providers consider a level below 300 as low. You might need to have more than one blood test to be sure. That's because testosterone levels vary throughout the day. The highest level is usually in the morning. That's when most health care providers like to measure it. Many men have low testosterone, but aren’t bothered by symptoms. Having a low level is not enough reason to start taking testosterone. But if you also have some of the signs and symptoms of low testosterone, it makes sense to consider testosterone therapy. The symptoms are: Less interest in sex Low mood Low energy Trouble concentrating and remembering Trouble getting an erection Less muscle Less body hair Decreased bone density More body fat High cholesterol Low number of red blood cells Testosterone Safety Even if you have low testosterone and some signs and symptoms, testosterone therapy might not be right for you. For some men, the risks are greater than the benefits. The biggest risk is for men who have prostate cancer, or a risk for prostate cancer, such as a family history. Testosterone can cause prostate cancer to grow. The same is true for male breast cancer. Other conditions also might keep you from taking testosterone. Ask your health care provider if testosterone is safe for you, if you have: Sleep apnea (loud snoring and trouble breathing at night) Polycythemia (too many red blood cells) Enlarged prostate gland Coronary heart disease What Else You Need to Know Ask your health care provider about all the possible side effects of testosterone therapy. These can include acne, fluid retention, and breast enlargement. Testosterone does not come in a pill. You need to take it by injection or applied on your skin. Injections can be painful and lead to temporary soreness. Patches or gels can cause skin reactions. You need to make sure the drug does not come into contact with preadolescent boys or pregnant women. Once you start taking testosterone, your health care provider will check your prostate every few months with a finger exam and a blood test called the PSA (prostate-specific antigen). You may also need to get an occasional blood test to make sure you are not making too many red blood cells. If you and your health care provider decide that testosterone therapy is right for you, benefits may include an increased interest in sex, more frequent erections, better mood, more energy, and stronger bones and muscles. Key Takeaways You need to have low testosterone and some signs or symptoms to benefit from testosterone treatment. You may not be able to take testosterone if you've been diagnosed with prostate or breast cancer. Testosterone can improve mood, energy, and sex drive for some men. Talk with your health care provider to find out if the benefits of testosterone therapy outweigh your risks.