If you’re an American man over age 40, chances are that you or someone you know is taking testosterone—but it’s not the wonder drug that manufacturers would have you believe. While it has benefits for those who are truly short on this vital hormone, a behavioral study done on healthy young men found a possible psychological downside to taking extra testosterone that goes beyond the known physical health risks.

It can lead you to make bad decisions.

Background: Testosterone replacement therapy has been approved by the FDA to treat a condition called hypogonadism, or low testosterone (low-T), that results from chemotherapy, injury or genetic conditions. But more and more, the hormone is being prescribed to men without these health issues as a way to boost libido, muscle mass and mood…and decrease fatigue and the risk for the bone-thinning disease osteoporosis. In fact, testosterone is prescribed to well over two million men in the US each year, and many of these prescriptions are for guys experiencing a normal age-related decline in the hormone, not truly low-T levels. This is an example of “off label” prescribing, which doctors are allowed to do, but it means that the drug is being given for a condition before the FDA has determined its effectiveness and safety for that condition.

Study details: Testosterone is indeed largely responsible for a man’s sex drive and muscle tone. And beyond testosterone therapy’s well-known physical health risks—including higher risk for heart disease and rising PSA numbers and possibly prostate cancer—it also has been associated with aggressive behavior and impulsiveness. Based on that, researchers in the area of cognitive psychology and decision theory set out to learn how testosterone might affect a man’s decision-making in particular.

They recruited 243 young men, mostly college students. On the morning of the study day, the men applied to their arms, shoulders and torsos either a single-dose topical testosterone gel or a placebo gel with no active ingredient. (The study did not look at using other forms of testosterone replacement, such as pills.) About five hours later—enough time for testosterone from the gel to peak in the men’s blood—the men took a cognitive test with three questions to measure their ability to make deliberate, reflective decisions. These brain teasers were designed to look simple but weren’t quite as simple as they looked—so to get them right, participants needed to suppress their first instincts and really think through the questions.

Here’s an example: A bat and a ball cost $1.10 in total. The bat costs $1.00 more than the ball. How much does the ball cost?

The findings: Were you quick to blurt out “10 cents!”?  That’s the answer the men using the testosterone gel were more likely to give than the other men—because they decided to trust the first answer that came to their minds. Correct answer: The ball costs 5 cents, and the bat costs $1.05. Overall, test results showed that the T-gel group made 20% more mistakes than those in the placebo group. That’s a pretty stunning result—just one application of supplemental testosterone gel given to those with otherwise normal testosterone levels reduced the likelihood of correct, analytic thinking when an easy but wrong solution jumped to mind!

How could the hormone cause impulsive and faulty decisions? One possibility is that testosterone inhibits prefrontal brain activity. The prefrontal cortex is involved in high-level planning, executive function and cognitive control, all of which are important for day-to-day decisions. An example of prefrontal brain activity at work is when you overcome the initial desire to have a cheeseburger and choose salad instead or when you get angry at someone but refrain from your first impulse to start shouting, notes Gideon Nave, PhD, one of the study authors. Inhibit prefrontal activity, and your decisions aren’t as well thought through.

While this study showed an interesting correlation between supplemental testosterone and decision-making, more research is needed to understand the full effect of testosterone replacement in older men who have naturally lower levels (a 1% drop every year after age 30 is normal)—remember, the study participants were young and did not have low T yet but were given extra testosterone. This study should prompt men considering testosterone-replacement therapy to have their testosterone levels tested to see whether they need replacement therapy and then, if so, to ask their doctors about the best dose for them and if the benefits outweigh the physical and cognitive risks.

Bottom line: This was only one study, but if you’re on testosterone therapy, it can’t hurt to take a mental inventory of whether you’re making too many rushed decisions, especially if they tend to be ones you later regret.

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