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By Deborah Kotz Globe Staff

Men may have seen ads asking whether they’ve “lost that loving feeling” and wondered whether they should get their testosterone levels tested. The promises made by pharmaceutical companies pitching testosterone creams or gels directly to men sound too good to pass up: bigger muscles, more energy, stronger sex drive. And prescriptions for the therapy have jumped more than 500 percent over the past two decades.

Whether men should get tested and treated for “low-T” -- as drug company ads call it -- remains controversial because of unknown risks of long-term testosterone therapy and hints that it could cause life-threatening health problems in older men who tend to have the lowest levels.

Researchers at the New England Research Institutes are conducting an observational study involving nearly 1,000 men over age 18 who are being treated with long-term testosterone therapy to boost low hormone levels in order to determine whether it raises their risk of prostate cancer.

A 2010 study published in the New England Journal of Medicine found that men over 65 years of age with low testosterone levels who were treated with a daily testosterone gel for six months gained muscle strength and endurance compared with those who used a placebo gel but also had more heart attacks. Out of 209 study participants, 23 in the testosterone group had a heart attack, a blocked artery that required treatment, or died of heart problems during the study, compared with 5 in the placebo group.