Almost one in ten HIV PrEP users report buying antibiotics for STI prevention, raising concerns over potential antibiotic resistance

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An online community survey conducted in the UK shows that 9% of HIV PrEP users are buying antibiotics to prevent themselves from getting STIs. The survey results from 2019 were published last month in the Sexually Transmitted Infections journal.

Those buying antibiotics were more likely to have had at least five condomless sexual partners in the last six months, more likely to have had a diagnosis of an STI within the last year and more likely to participate in chemsex. This suggests that PrEP users with risk factors for getting STIs are recognising that they are at increased risk and buying antibiotics to try to prevent infection before having any symptoms or a positive test for an STI.

Previous small studies have shown that the antibiotic doxycycline can be used to prevent STIs, in a similar way to how PrEP is used to prevent HIV. Doxycycline has been shown to decrease the rates of STIs in HIV-positive gay and bisexual men and HIV-negative PrEP users. Doxycycline is an effective treatment for chlamydia and is also used to treat syphilis in people who are allergic to penicillin antibiotics.



Antibiotics, also known as antibacterials, are medications that destroy or slow down the growth of bacteria. They are used to treat diseases caused by bacteria.


The result of a statistical test which tells us whether the results of a study are likely to be due to chance and would not be confirmed if the study was repeated. All p-values are between 0 and 1; the most reliable studies have p-values very close to 0. A p-value of 0.001 means that there is a 1 in 1000 probability that the results are due to chance and do not reflect a real difference. A p-value of 0.05 means there is a 1 in 20 probability that the results are due to chance. When a p-value is 0.05 or below, the result is considered to be ‘statistically significant’. Confidence intervals give similar information to p-values but are easier to interpret. 


Having sex without condoms, which used to be called ‘unprotected’ or ‘unsafe’ sex. However, it is now recognised that PrEP and U=U are effective HIV prevention tools, without condoms being required. Nonethless, PrEP and U=U do not protect against other STIs. 


The use of recreational drugs such as mephedrone, GHB/GBL and crystal meth before or during sex.


Chlamydia is a common sexually transmitted infection, caused by bacteria called Chlamydia trachomatis. Women can get chlamydia in the cervix, rectum, or throat. Men can get chlamydia in the urethra (inside the penis), rectum, or throat. Chlamydia is treated with antibiotics.

However the British Association for Sexual Health and HIV advises against the use of antibiotics for prevention of infections before diagnosis, due to concerns about antibiotic resistant bacteria. Gonorrhoea has high levels of resistance to doxycycline, so doxycycline would be unlikely to be effective at preventing gonorrhoea infections. Doxycycline and similar antibiotics are used to help treat mycoplasma genitalium, but there are already major concerns around antibiotic resistance when treating this STI.

The data was taken from the PrEP Users Survey, an online survey carried out by community organisations PrEPster and iwantPrEPnow alongside Public Health England, to collect information about PrEP use in the UK. In the 2019 survey, the question "Do you buy antibiotics to prevent STI infections, either privately or through the internet?" was added. Participation was encouraged by social media, dating apps and the PrEPster and iwantPrEPnow mailing lists.

Overall, 1856 PrEP users were identified with 98% identifying as male, 93% as gay, 85% as white and 63% over 35. Although responses were encouraged across the UK, 48% overall were from London. Importantly, 95% reported accessing a sexual health clinic within the last year and were likely to have interacted with a healthcare professional.

In the survey, 9% (167/1856) of PrEP users used antibiotics to prevent STIs, compared to 91% (1681/1856) who did not.

From the data, 72% of those using antibiotic prevention had condomless sex with five or more partners compared to 41% of those who did not buy antibiotic prevention (p value < 0.001). There was also a higher percentage of those taking drugs associated with chemsex in the group buying antibiotics (33%) than in the group who weren't (22%, p value < 0.001). Finally the group buying antibiotic prevention were significantly more likely to have had a diagnosed STI in the last year (63%) than the group who were not taking antibiotics (48%, p value < 0.001).

In both groups, 75% used daily PrEP as opposed to other methods of taking PrEP, such as event-based PrEP. There was no statistically significant relationship found between how PrEP was accessed by participants and their likelihood of using antibiotic prophylaxis, so those already buying PrEP drugs on the internet were not more likely to also be buying antibiotics.

The survey did not identify exactly which antibiotics were being bought, nor how they were being used. Information around why STI prevention was being used and how long it was used for was not gathered either. The question in the survey asked about buying antibiotics, so excludes those who may be using antibiotic STI prevention but not directly buying it themselves. It is also not possible from this data to know about STI prevention in gay communities not taking PrEP, nor STI prevention use in other populations taking PrEP. The majority of participants were white and over the age of 35.

"Those buying antibiotics had more condomless sexual partners, were more likely to have a recent diagnosis of an STI and were more likely to participate in chemsex."

However, the investigators believe that this is the largest study to date in this area. Studying the use of antibiotics in gay and bisexual men who access PrEP is necessary, given that there is evidence that PrEP has been linked to increased rates of rectal STIs. This survey suggests that gay and bisexual men using PrEP who also use antibiotics to prevent STIs have a higher likelihood for other risk factors for STIs compared to those who do not use antibiotics for prevention.

The researchers are concerned that some PrEP users using antibiotics as prevention for STIs may not disclose this to healthcare professionals, as it is not recommended by UK guidelines. This may lead to a lack of correct information and increase the risk of antibiotic resistance because the wrong antibiotics may be used. Similarly, advice around re-testing to check an STI has been cured and which antibiotics can be used against difficult to treat infections cannot be given if people are treating themselves at home without consulting a healthcare professional. The data suggests that antibiotics are being bought but there are still lots of unknowns about antibiotic prevention for STIs, so further research on this subject is needed.

As the researchers stated: "Awareness and public health education on the informal use and abuse of antibiotics, for both patients and providers, would be beneficial."


O'Hallaran C et al. Factors associated with reporting antibiotic use as STI prophylaxis among HIV PrEP users: findings from a cross-sectional online community survey, MayJuly 2019, UK. Sexually Transmitted Infections, online ahead of print  20 October 2020.