Taking medicine to prevent HIV might seem a good reason to feel confident about being able or eligible to give blood. Yet people taking pre-exposure prophylaxis, known as Prep, cannot donate while using it, and must wait for three months after their last dose before doing so.
Author
- Jaid Debrah-Donkor
PhD Candidate, Blood Safety & Virology (PrEP/GBMSM), University of Oxford
Prep is taken by people without HIV to reduce their chance of acquiring the virus. According to the NHS, daily Prep is around 99% effective when taken correctly .
But if someone acquires HIV while taking Prep, the medicine can make the infection harder to detect. If an infected donation passed through testing undetected, it could transmit HIV to someone receiving the blood. Donation rules provide an additional safeguard against that possibility.
Research I carried out with colleagues found drugs used in Prep in some blood donations in England. These medicines have other uses, so their presence cannot confirm why someone took them. Our findings also cannot establish whether donors understood the restrictions, and we did not demonstrate HIV transmission through donated blood.
Our findings raise a question for blood services: how can they explain the restrictions clearly while supporting people's use of effective HIV prevention?
Why Prep can make HIV harder to detect
If someone acquires HIV while taking Prep, the medicine can limit the virus's ability to multiply, reducing the amount in their blood. It can also delay or weaken the production of antibodies, proteins the immune system makes in response to infection.
Blood services test donations for signs of HIV. These include genetic material from the virus and antibodies produced by the body. Research examining blood samples from studies of Prep shows that these medicines can make an infection harder to detect. Someone may have HIV before the tests can reliably identify it.
This helps explain why blood services ask questions about a person's health and medication as well as testing their blood. Those answers provide information that a test result alone cannot supply.
Under UK blood donation guidelines , people must wait three months after their last Prep tablet before donating. For Prep given by injection, the waiting period is two years after the last injection. People must also meet the other conditions for donating blood.
Long-acting injectable Prep can remain in the body for months after injections stop. The waiting periods apply regardless of sexuality, even when someone would otherwise be able to donate.
How donation rules have changed
Blood donation rules have become more inclusive. In 2021, the UK introduced individual assessments following a review known as Fair (For the Assessment of Individualised Risk).
Previously, men who had sex with men faced restrictions that applied to them as a group. Under the revised approach , donors are asked the same questions about recent sexual behaviour, regardless of gender. This allows more gay and bisexual men to donate while maintaining safeguards for people receiving blood.
For example, anyone who has had anal sex with a new partner or multiple partners must wait three months after the last such contact before donating. Recent Prep use remains a separate reason why someone must wait.
Someone taking Prep might understandably feel reassured by its effectiveness and assume that tests on donated blood provide an additional safeguard. Explaining how the medicine can affect those tests should therefore be part of explaining the donation rules.
What our research found
We tested blood samples collected in 2023 from 84 male donors in England. Their blood showed evidence of current or past syphilis, a sexually transmitted infection. Six samples, or 7.1%, contained both medicines used in a common type of Prep tablet.
The same medicines can also be used to prevent HIV after a possible exposure, or as part of treatment for someone who already has HIV. Finding them does not establish that a donor has HIV.
Our wider analysis found that donations showing evidence of current or past syphilis increased from about four to ten per 100,000 donations between 2018 and 2024. This does not establish that Prep or changes to donation rules caused the increase.
Because we tested a particular group of donors, these findings cannot tell us how common Prep use is among everyone who gives blood. Nor can they tell us whether people knowingly disregarded donation rules or misunderstood them.
Making the guidance easier to understand
The reason for the waiting period should be explained wherever people receive information about Prep and wherever they check whether they can donate blood. Knowing how a medicine can affect test results may make the restriction easier to understand.
Blood services should also make it straightforward for people to ask questions privately. Donors need to feel able to discuss medication and sexual health without expecting to be judged.
The different waiting periods for Prep tablets and injections make clear guidance especially important. Someone familiar with the three-month wait after taking tablets could otherwise assume that the same rule applies to injections.
People should not stop Prep simply to become eligible to donate blood. Anyone considering a change should discuss it with their doctor or nurse at a sexual health clinic, as stopping removes its protection against HIV .
Prep remains a major advance in HIV prevention. Explaining why people taking it need to wait before donating can help protect those receiving blood while supporting people's use of medicine that protects their own health.
Jaid Debrah DPhil Student in Medical Sciences University of Oxford/ NHSBT BTRU-GEMS (Blood and Transplant Research Unit- Genomics to Enhance Microbiological Screening. She receives funding from NIHR. She is affiliated with The University of Oxford, NHSBT, UKHSA, and International Society of Blood Transfusion.
