For PrEP to be most effective, the medicine needs to be at a certain level in your body. As the body takes time to absorb this medicine, PrEP needs to be taken before exposure to HIV and afterwards.
Once you start taking PrEP, you need to be tested and monitored regularly for HIV and STIs.
Testing and monitoring when taking PrEP
How you take PrEP depends on your circumstances.
There are 2 ways to take it:
- daily dosing - you take PrEP every day
- event-based dosing (EBD) - you take PrEP around the time of sex
Note: throughout this page the term vagina includes neovagina.
Daily dosing
For receptive anal, insertive anal or vaginal/frontal sex
When you start PrEP, take 2 tablets (double dose) at least 2 hours but no more than 24 hours before sex. This is to make sure that the medicine levels are high enough to be protective before condomless sex.
Continue to take 1 tablet a day, every day.
To stop PrEP, take 1 tablet every 24 hours until 2 tablets have been taken after your last sexual exposure.
For receptive vaginal/frontal sex and sharing injecting drug equipment
When you start PrEP, take 2 tablets (double dose) at least 2 hours but no more than 24 hours before exposure. This is to make sure that the medicine levels are high enough to be protective before exposure.
Continue to take 1 tablet a day, every day.
To stop PrEP, take 1 tablet every 24 hours until 7 tablets have been taken after your last exposure.
Tips on how to take daily PrEP
- Use a pill box - this makes it easy to see whether you have taken or missed a dose.
- Set a reminder alarm on your phone or use a pill reminder app.
- Pick a regular time to take your PrEP and try to stick to this each day - link it to a routine task like brushing your teeth.
- If you have a break from PrEP and have condomless sex during this time, consider PEP and have a HIV test before you restart PrEP.
Event-based dosing (EBD)
For receptive anal, insertive anal or vaginal/frontal sex
Studies show that EBD is as effective as daily dosing for people having anal sex.
EBD is a way of using PrEP only when you are likely to have sex.
EBD is an option for people who:
- do not want to take PrEP all the time
- only occasionally have sex without condoms
- can plan for sex in advance
Transgender women who are taking gender-affirming (feminising) hormone therapy should only use EBD while under the care and supervision of a consultant in genitourinary medicine or infectious diseases.
EBD is not suitable:
- if you have hepatitis B - it is very important to know your hepatitis B status before taking PrEP
EBD for sexual episodes over one day
- Take 2 tablets (double dose) at least 2 hours but no more than 24 hours before sex. This is to make sure that the medicine levels are high enough to be protective when you have sex.
- Take a single tablet 24 and 48 hours after the double dose.
This is often called ‘PrEP 2-1-1’ or ‘PrEP on demand’.
EBD for sexual episodes over more than one day
Use this schedule if you may have sex several times over a period of time. For example, you are away for a weekend or on holidays.
- Take 2 tablets (double dose) at least 2 hours but no more than 24 hours before sex.
- If you are continuing to have sex, take 1 tablet 24 hours after the double dose.
- Continue taking 1 tablet every 24 hours for the days you are having sex.
- Take 1 tablet for 2 days after last having sex.
Do not miss any doses if you are taking event-based PrEP.
Missed doses and when to take PEP
If you miss some PrEP doses you may need to take post-exposure prophylaxis (PEP) to reduce the risk of becoming HIV positive.
PEP is a 28-day course of medicine that helps reduce your risk of getting HIV after exposure to the virus.
Oral sex only
If your only exposure has been through oral sex, you will not need to take PEP. This is regardless of how many doses you missed.
Daily PrEP for anal and insertive vaginal/frontal sex
If it has been more than 7 days since your last PrEP dose:
- restart PrEP with 2 tablets (double dose) as soon as possible, preferably within 24 hours and no later than 72 hours after the sexual exposure
- continue taking 1 tablet a day
- seek an urgent PEP assessment as soon as possible within 72 hours - whether you have any PrEP medicine or not
If it has been 7 days or less since your last PrEP dose:
- resume daily PrEP as soon as possible
- if you do not have any PrEP medicine, seek an urgent PEP assessment as soon as possible within 72 hours
Daily PrEP for receptive vaginal/frontal sex
If it has been more than 3 days since your last PrEP dose:
- restart PrEP with 2 tablets (double dose) as soon as possible, preferably within 24 hours and no later than 72 hours after exposure
- continue taking 1 tablet a day
- seek an urgent PEP assessment as soon as possible within 72 hours - whether you have any PrEP medicine or not
If it has been 3 days or less since your last PrEP dose:
- restart PrEP with 2 tablets (double dose) as soon as possible
- continue taking 1 tablet a day
- contact your PrEP service
- if you do not have any PrEP medicine, seek an urgent PEP assessment as soon as possible within 72 hours
Daily PrEP for sharing drug-injecting equipment
If it is more than 4 days since your last PrEP dose:
- restart PrEP with 2 tablets (double dose) as soon as possible within 24 hours and no later than 72 hours after the exposure
- continue taking 1 tablet a day
- seek an urgent PEP assessment as soon as possible within 72 hours - whether you have any PrEP medicine or not
If it has been 4 days or less since your last PrEP dose:
- restart PrEP with 2 tablets (double dose) as soon as possible
- continue taking 1 tablet a day
- contact your PrEP service
- if you do not have any PrEP medicine, seek an urgent PEP assessment as soon as possible within 72 hours
EBD for anal sex and insertive anal, vaginal/frontal sex
If you miss the first after-sex dose of event-based PrEP and it’s under 48 hours since exposure (and you took at least 1 tablet before sex):
- take the first after-sex dose
- take the second after-sex dose 24 hours after the first to complete the course
If you miss the first after-sex dose of event-based PrEP and it’s more than 48 hours since exposure:
- take the first after-sex dose
- seek advice about PEP within 24 hours
If you do not have any PrEP medicine, seek an urgent PEP assessment as soon as possible within 72 hours.
Getting PEP urgently
All PrEP services must have PEP or have access to a service that has PEP, should you need it urgently.
Where to get PEP - sexualwellbeing.ie
Changing how you take PrEP
You can change how you take PrEP or stop and restart as your circumstances change.
Talk to your healthcare professional if you are taking daily PrEP and want to switch to EBD. They will help you decide if EBD is an option for you. They will help you make the change in schedule safely.
If you are taking EBD and want to switch to daily dosing:
- start with a double dose - take the medicine at least 2 hours and no more than 24 hours before you have sex
- continue taking PrEP daily
If you switch from EBD to daily dosing, make sure you have enough medicine to last until your next appointment with your PrEP service.
In some situations, your healthcare professional may suggest switching from daily to EBD to lessen the impact of the PrEP medicine on your kidneys.
Deciding to stop PrEP
Other considerations
Contraception
It is safe to use PrEP with most hormonal contraception (ring, patch, the pill, or an implant). PrEP will not affect your contraception and your contraception will not affect PrEP.
PrEP in pregnancy
There are few studies on the use of PrEP in pregnancy. But no PrEP-related pregnancy complications have been identified.
Untreated HIV in pregnant people is associated with an increased risk of transmission to the baby. You may be advised to take PrEP in pregnancy where there is a risk of HIV.
If you are taking PrEP and could become pregnant, talk to your healthcare professional. They can help you make decisions about what is best for you.
Drug resistance
If you get HIV while taking PrEP, there is a small risk of developing resistance to one or both PrEP medicines. This means they will not work as well against HIV.
PrEP studies have shown that very few people got HIV while taking PrEP. In those who did, fewer than 1 in 20 developed resistance to the medicine.
The possibility of drug resistance increases if you:
- start PrEP without knowing that you have HIV - always have a laboratory HIV blood test before you start PrEP
- take a break from PrEP and do not check your HIV status before starting it again
- do not take PrEP correctly, get HIV and continue to take some PrEP medicine
- are exposed to drug-resistant HIV - this is very rare
Having a laboratory HIV blood test
