That’s the conclusion of a new study that is going to make life much harder for Merck to wring needed sales out of its controversial HPV vaccine. The study, which appears in the New England Journal of Medicine, comes as the drugmaker is already struggling to convince college-age and older women to get the vaccine, which costs about $360 for a three-dose regimen.

The vaccine, which is approved for girls and young women ages 9 to 26, makes economic sense for preteens because they are less likely to have the sexually transmitted virus that causes cervical cancer, according to the study. But the cost-benefit depends on how long Gardasil’s protection will last, although Merck contends the vaccine is cost effective for women through age 24.

The analysis predicted that life expectancy gained by giving Gardasil to women older than 18 doesn’t outweigh the expense. “We aren’t saying older women can’t benefit. We are just saying that from a societal perspective there might be a better use of this investment in money,” Jane Kim, an author on the study and an assistant professor of health decision science at Harvard University, tells Bloomberg News. “You are getting diminishing returns.”

The researchers compared the price of the vaccine and other expenses involved in getting the shot along with regular pap smears to the cost of pap smears. A pap smear, which detects abnormal cell change that could signify cervical cancer, costs about $38.68 and is recommended at least once every three years for women ages 21 through 64, according to the National Institutes of Health.

To wit, Gardasil cost about $43,600 per “quality-adjusted life year” gained when administered to 12- year-old girls. This falls below the $50,000 per QALY benchmark used by many researchers as a maximum for cost-effectiveness, although other researchers cite $100,000 per QALY.

It would cost $97,300 per QALY, however, to vaccinate girls and women through age 18; $120,400 per QALY for girls and women up to age 21, and $152,700 for girls and women up to age 26. However, the cost becomes more attractive when considering protection against genital warts - the cost per QALY when given to 12-year-olds falls 20 precent to $34,900, and to $133,600 for a program through age 26.

Current data has only looked at Gardasil’s ability to stop HPV for up to five years. It is unknown whether Gardasil can reduce overall rates of cervical cancer and deaths, Charlotte Haug, editor-in-chief of the Journal of the Norwegian Medical Association, wrote in a separate editorial also published in the New England Journal.

“There is good reason to be cautious about introducing large-scale vaccination programs,” Haug tells Bloomberg. “Instead, we should concentrate on finding more solid answers through research rather than base consequential and costly decisions on yet unproven assumptions.”

It is routine for vaccines to be used after five years of efficacy data and it would be unfair to patients keep the product off the market while researchers study its long-term effects, Rick Haupt, Merck’s executive director of Gardasil clinical research, tells Bloomberg. Merck may present data this year on the effects of the vaccine over nine years.

Gardasil protects against the HPV strains 16 and 18, which are responsible for 70 percent of cervical cancers, and HPV strains 6 and 11, which cause 90 percent of genital warts. While many of the sexually transmitted infections caused by HPV are resolved naturally, others develop into genital warts and cervical cancer. The CDC recommends girls be vaccinated at age 11 or 12.