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The spread of some sexually transmitted infections could potentially be dramatically reduced by instructing people who have had unprotected sex to take antibiotics within 24 hours after the intercourse, a new study suggests.

But such a strategy, which was tested in a population of men who had frequent unprotected sex with a number of male sex partners, could spark a controversy over the use of antibiotics and the general threat of growing antibiotic resistance.

“My message with that study would be that we need to do more research to prevent STIs — because that’s a concern. And this strategy … could potentially be used,” said Dr. Jean-Michel Molina, head of infectious diseases at Saint-Louis Hospital in Paris and the lead author of the study.


Molina insisted he would never support long-term use of antibiotics to prevent STIs, but said that the approach might be an effective short-term strategy when paired with other control approaches, like more frequent STI testing among high-risk populations.

“I don’t want this strategy to be used widely in any person, clearly,” Molina said. “But if you can select a group with a high incidence rate of syphilis or chlamydia, and you want to try to reduce the rate of syphilis quite quickly in this group of people, you may think that this strategy could be used for a couple of months.”


Rates of syphilis in particular have risen steadily in recent years; the rate in 2015-2016 — 8.7 cases per 100,000 people — was the highest since 1993, the Centers for Disease Control and Prevention reported. The rate of infection increased in every age group over the age of 15, in both men and women, and in all ethnic groups.

Molina’s study was recently published by the journal Lancet Infectious Diseases. The Bill and Melinda Gates Foundation was one of its funders.

The researchers randomly assigned 232 men to one of two groups. One of the groups was given antibiotics — the drug doxycycline — to take if they had unprotected sex. They were told to take two pills per encounter, and no more than six in a week, ideally within 24 hours and no later than 72 hours after the intercourse. In reality, the median use among the men was about 6.8 pills per month.

The men were tested regularly for STIs, and in the nine or so months they were followed, the rates of some sexually transmitted infections fell dramatically in the treatment group. The overall reduction of all STIs was 47 percent, but that average was dragged down by the fact that doxycycline doesn’t cure gonorrhea.

There was, however, a 70 percent reduction in chlamydia infections and cases of syphilis decreased by 73 percent. The authors noted, however, that the study length was short and they can’t tell if the strategy would work as well over the long term.

The results can’t be taken in isolation, others experts warned. Using doxycycline this way could drive the bacteria that cause the infections to develop resistance to the drug, warned Dr. Brad Spellberg, an infectious diseases specialist and chief medical officer at the Los Angeles County-University of Southern California Medical Center in Los Angeles.

Spellberg also said people who used the drug this way would be exposing the bacteria they have in their gastro-intestinal tracts — their own gut flora — to regular doses of doxycycline. And those bacteria too could develop resistance to the drug, leading to a host of other health problems.

“You’re causing friendly fire injury,” he said.

The full picture of the knock-on effects of exposing your gut flora to antibiotics is still coming into focus, said Ramanan Laxminarayan, director of the Center for Disease Dynamics, Economics, and Policy and a leading voice on the dangers of mounting antibiotic resistance. He noted a recent report in the journal Science revealed that some cancer immunotherapy drugs worked less well in people who had recently taken antibiotics.

Molina noted that antibiotic resistance to doxycycline has not been seen in chlamydia or syphilis, despite the fact that the drug has been used to treat these infections for decades. Still, he said the possibility it could arise cannot be discounted.

A commentary published with the study argued that reducing infection rates in men who have sex with men and who are highly sexually active might lower STI infection rates more generally in a community.

Authors Christopher Fairley and Eric Chow, of Australia’s Melbourne Sexual Health Center at Monash University, said pressure from patients to be given doxycycline on a preventative basis might be substantial, given that the drug is also prescribed in six-month courses to treat acne. Like Molina and his co-authors, they said STI preventive use at this point is premature.

Laxminarayan did not dismiss the notion of this type of use out of hand, however. “I certainly think that for a small subset of the population, if this helps prevent syphilis, then it certainly is worth exploring further,” he said.

But he said additional studies should be conducted first to try to get a better picture of the potential consequences of using doxycycline this way, including the risk of resistance developing, the potential that it might further erode condom use, and any other unintended consequences.

  • Nicholas: If 50% are over 70, then 50% are younger than 70. When I go with my sister who gets Chemo the place is not overcome by older people. Lots of 30s & 40s. The current playbook blames the lifestyle of the victims(victim shaming). Hogwash! I know many who live a healthy life and they have cancer. It is all smoke and mirrors for continuing to do little about Prevention! No vaccine and few if any ads by CDC NIH etc targeting young people about limiting sex partners and living healthy to keep one’s immune system functioning well.

  • Nicholas:
    I do not think it it a matter of technology. There is ample experience with vaccines including HPV (not a herpes virus). What is needed is the will to investigate and develop one.

    Last statistics I saw from 5 or 6 years ago estimated 8 million people die from cancer worldwide. … Lung, liver, stomach, and bowel are the most common causes of cancer accounting for nearly half of all cancer deaths. I am sure the numbers are larger now. It is all around me. When I was a kid cancers were rare. Now it’s the new chronic disease. Very scary
    Enjoy your holidays

    Treating cancer is big business because Big Pharma is the player. We all know the pricetags on drugs. Like I said, I wouldn’t hold my breath waiting.

    • Carol,
      The technology makes an immense difference. A shorter development time means more profit potential, which is the key incentive. Regulatory costs remain an immense barrier to product development.

      Total cancers have increased over time because it is mostly a disease of aging, and people are living much longer. Cancer has been in decline in the US, mostly due to a reduction in smoking. The HPV vaccine will move the numbers in a positive direction, one hopes.

      Behavioral changes are the best hope for reducing the risks of many cancers. Don’t smoke or become obese, stay active, eat nutritionally dense foods, don’t drink too much alcohol, don’t wait too long to have a child and breastfeed it.

      “Cancer is primarily a disease of older people, with incidence rates increasing with age for most cancers. In the UK in 2012-2014, on average each year half (50%) of cases were diagnosed in people aged 70 and over.”

      Happy holidays.

  • Nicolas: I do know it has been proven that EBV is causative but I don’t know if anyone has zeroed-in on any one particular strain. All the other viruses in the Herpes group have a role in cancer. When the human papilloma virus (HPV) was discovered to be causative of throat, tongue, anus, vulva, cervix, it took over 30years for a vaccine! There is not a lot of interest in vaccines because they are not money-makers. Pretty sad. Don’t expect a accine anytime soon.

  • No sidestep intended. EBV is endemic in our society, perhaps even worldwide. It is easily transmitted by kissing, intimate or not. Parents can transmit EBV to their kids by simply kissing them on the mouth. However, viral load may make a difference because of cell availability. It is higher in sperm and vaginal fluids. All Viruses need living cells in which to live & reproduce. There may be studies out that discuss viral load via a vis infectivity. Likely there are several different strains of EBV and some may be more oncogenes than others.

    • In the event that strains are identified that are proven causes of cancer, we can hope that a vaccine is developed. Until then almost all of us live with EBV. Much the same can be said of CMV, which infects up to 80 percent of people in the United States.

  • No antibiotic will protect these individuals from onco-viruses they are likely to acquire through unprotected sex- viruses known to cause cancer in susceptible individuals, and to be passed down to offspring via sperm and egg. Yes, the children do suffer from the deeds of the father.

    • MPWH
      Certain members of Herpes group of viruses are known to be causative for certain cencers. These are not “managed” on a molecular level. The only thing managed is the physical flare of some, but the underlying damage,if any, may have already occurred. Unprotective sex is nothing to take lightly. Health departments are more than remiss in not sharing this information.

    • Nicolas, please read, “The Story of Epstein-Barr Cancer Virus” by Crawford and Rickinson. “Epstein” is in the Herpes group. It is found to be causative in a number of cancers Herpes virus #8 causes Kaposi’s Sarcoma (Cancer), Burkitts lymphoma (Cancer) as well. Other viruses HPV and Retroviruses ( not AIDS) are causative in certain leukemias and lymphomas). These are all sexually transmitted because viruses can only live and multiply within living cells.

    • Nicolas, it is also transmitted by sexual activity of all kinds especially intercourse. EBV is not only “linked” it is causative. It is very difficult to prove virus as causative because Koch’s postulates can’t be used. The postulates are OK when the culprit is a bacterium…it is not ethical to infect someone with an oncovirus like certain strains of EBV and others to see what happens. Merry Christmas!

    • Carol,
      I basically agree except with your assertion that EBV is “especially” transmitted through intercourse. You sidestepped my point that EBV is commonly transmitted through intimate kissing, and that there is no way to prevent that. According to Duke researchers in 2010, “About 90 percent of people are infected at some time in their lives with Epstein-Barr virus (EBV), usually with no ill effects.”

      We at least know that 90 percent of the population doesn’t develop an EBV-related cancer. There is no realistic chance that the high EBV infection rate will be reduced. Kissing will not go out of style. People didn’t stop having risky sex when syphilis was widespread and untreatable.

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