Testosterone products can increase the risk of venous blood clots. Manufacturers of such products are now required to include related information and warnings on labels.

Previously blood clots were listed on labels as warnings under polycythemia, an increase in red blood cells. Studies have shown that this venous abnormality can sometimes be the result of testosterone products.

However, in response to reports of blood clots unrelated to polycythemia, the U.S. Food and Drug Administration (FDA) is requiring manufacturers to list risk of blood clots as a direct warning on testosterone products, rather than only as an effect of polycythemia. The FDA received such reports after the approval and sale of these products.

The products in question are useful for men with low testosterone levels, sometimes a result of chemotherapy and sometimes genetics. Decreased levels can lead to symptoms such as decreased libido, depression and fatigue.

A blood clot can become a thrombus, which is a clot that stays in place, or an embolism, which is a clot that travels to other parts of the body and blocks blood flow in those locations.

The FDA is also conducting an investigation of the risk of stroke, heart attack, and death following usage of testosterone products. The investigation, which began in January, was influenced by two separate studies showing significantly higher risks of cardiovascular problems in men using testosterone products. The men in the studies were in both the older, post-60 age group as well as the younger, pre-30 group. Officials say this investigation is unrelated to the venous blood clots and the new requirements for manufacturers.

The FDA said in its statement explaining its investigation that doctors "should consider whether the benefits of FDA-approved testosterone treatment is likely to exceed the potential risks of treatment."

It also stressed that patients should not stop taking the prescribed products without consulting their healthcare professionals. All products, which are sold as topical gels, transdermal patches, and injections, are FDA-approved only as a therapy for low testosterone levels seen in conjunction with a medical condition.

According to Bloomberg, testosterone drugs have a $1.6 billion market value.