The marketing of “low T” as a common medical condition helped propel sales of testosterone gels, patches, injections and tablets to about $2 billion in the United States last year, according to IMS Health, a health care information company. “The big thing is, we just don’t know the long-term risk of testosterone therapy at this time,” says Dr. Jacques G. Baillargeon, an epidemiologist at UTMB who has studied testosterone-prescribing trends in the United States. “It’s particularly concerning when you see the dramatic increase happening at such a large scale so quickly.”