For PrEP to be most effective, the medicine needs to be at protective levels at the time that HIV exposure may happen. As the body takes a while to absorb medicines, this means PrEP needs to be taken both before exposure and afterwards.
Current guidelines recommend that PrEP provides protection against HIV after a double dose (2 tablets) taken between 2 and 24 hours before protection is needed. This applies to different types of sex and where protection is needed from injecting drug use. The duration of PrEP after an episode of sex or injecting drug use varies. In the following sections we will look at different circumstances and how to safely start and stop PrEP.
Once you start taking PrEP, you need to be tested and monitored regularly. Read about why being tested for HIV and STIs is important when you are taking PrEP.
There are 2 different ways you can take PrEP, depending on your circumstances:
- daily PrEP (taking PrEP every day)
- event based dosing (taking PrEP around the time of sex)
Starting and stopping PrEP safely
Daily PrEP dosing schedule: for receptive anal, and insertive anal, vaginal and neovaginal sex
Starting: When you start taking PrEP, you need to take 2 tablets of PrEP at least 2 hours (but no more than 24 hours) before sex. This is to make sure that the drug levels are high enough to be protective before condomless sex.
Continuing: Continue taking 1 tablet of PrEP per day, every day.
Stopping: To stop PrEP, you must continue to take 1 tablet every 24 hours until 2 tablets have been taken after your last sexual exposure.
Daily PrEP dosing schedule: for receptive vaginal/neovaginal sex and sharing injecting drug equipment
Starting: When you start taking PrEP, you need to take 2 tablets of PrEP at least 2 hours but no more than 24 hours before exposure, to make sure that the drug levels are high enough to be protective before the exposure.
Continuing: Continue taking one tablet of PrEP per day, every day.
Stopping: To stop PrEP, you must continue to 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 an alarm on your phone or use a pill reminder app to remind you.
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. It doesn’t have to be the exact same time but it will help get you into a routine.
If you have a break from PrEP and have condomless sex during this time, it is important to consider PEP and have another HIV test before you re-start PrEP.
Event based dosing (EBD): for receptive anal and insertive anal, vaginal, neovaginal sex
This is also known as ‘PrEP 2-1-1’ or ‘PrEP on demand’
Take 2 pills before sex as a double dose and a single pill 24 and 48 hours later.
Several studies mainly in gay men have shown that EBD is as effective as daily PrEP 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
Event based dosing schedule: for sexual episode(s) over one day
- Take 2 tablets of PrEP (double dose) at least 2 (but no more than 24 hours) before sex. The before-sex double dose is very important to make sure that there is enough drug in your body when you have sex.
- Take a single tablet 24 hours after the double dose.
- Take a final tablet 24 hours later. This is often referred to as 2-1-1.
Event based dosing schedule: for sexual episodes over more than one day
For example, where you are away for the weekend or on holidays where you may have sex several times over a period of time.
- Take your double dose (2 tablets of PrEP) at least 2 (but no more than 24 hours) before sex.
- If you are continuing to have sex, take 1 tablet 24 hours after the double dose and continue taking 1 tablet every 24 hours for the days you are having sex.
- Continue taking PrEP until you have taken 2 doses after your last sex. This means taking 1 dose on each of the 2 days after your last sex.
It is important not to miss doses if you are taking event based PrEP.
Missed doses and the need for PEP
If you have missed some PrEP doses you may need to start on PEP to reduce the likelihood of becoming HIV positive.
Oral sex only
- If the only exposure has been through oral sex, regardless of the number of missed doses of PrEP, PEP is not indicated.
Daily dosing: condomless anal sex (insertive/receptive) and insertive vaginal/neovaginal sex
- If it has been more than 7 days since your last oral PrEP dose:
- you need to restart PrEP with a double dose (2 tablets) as soon as possible, preferably within 24 hours and no later than 72 hours after the sexual exposure. Continue taking one tablet of PrEP daily and seek an urgent PEP assessment as soon as possible within 72 hours.
- if you do not have any PrEP medication, seek an urgent PEP assessment as soon as possible within 72 hours.
- If it has been 7 days or less, since your last oral PrEP dose:
- you need to resume daily PrEP as soon as possible
- if you do not have any PrEP medication, seek an urgent PEP assessment as soon as possible within 72 hours.
Daily dosing: condomless receptive vaginal/frontal, neovaginal sex
- If it is more than 3 days since your last oral PrEP dose:
- you need to restart PrEP with a double dose (2 tablets) as soon as possible, preferably within 24 hours and no later than 72 hours after the sexual exposure. Continue taking one tablet of PrEP daily and seek an urgent PEP assessment as soon as possible within 72 hours.
- if you do not have any PrEP medication, seek an urgent PEP assessment as soon as possible within 72 hours.
- If it has been 3 days or less since your last oral PrEP dose:
- you need to resume PrEP with a double dose (2 tablets) as soon as possible. Continue taking one tablet of PrEP daily and make contact with your PrEP service.
- if you do not have any PrEP medication, seek an urgent PEP assessment as soon as possible within 72 hours.
Risk through sharing injecting drug equipment
- If it is more than 4 days since your last oral PrEP dose:
- You need to restart PrEP with a double dose (2 tablets) as soon as possible within 24 hours and no later than 72 hours after the exposure. Continue taking one tablet of PrEP daily and seek an urgent PEP assessment as soon as possible within 72 hours.
- if you do not have any PrEP medication, seek an urgent PEP assessment as soon as possible within 72 hours.
- If it has been 4 days or less since your last oral PrEP dose:
- you need to resume PrEP with a double dose (2 tablets) as soon as possible. Continue taking one tablet of PrEP daily and make contact with your PrEP service.
- if you do not have any PrEP medication, seek an urgent PEP assessment as soon as possible within 72 hours.
Event-based dosing (EBD) for condomless anal sex and insertive anal/vaginal/neovaginal sex
- If you have missed the after-sex dose for event-based PrEP:
- if you are late with, or missed, the first after-sex dose, this can still be taken up to 48 hours after sex, provided you took at least one tablet before sex; the second after-sex dose should be taken 24 hours after the first to complete the course.
- if it is more than 48 hours after your last sexual exposure, the first after-sex dose should be taken and you should seek advice within 24 hours regarding PEP.
- if you do not have any PrEP medication, seek an urgent PEP assessment as soon as possible within 72 hours.
All PrEP services are required to either have PEP or have an established referral pathway to a service that has PEP, should you need to access it urgently.
The list of locations where PEP is available in Ireland is available here: https://www.sexualwellbeing.ie/need-urgent-information-and-care-/post-exposure-prophylaxis/.
Changing how you take PrEP
Your need for PrEP and protection against HIV may change over time. You can change how you take PrEP or stop and restart PrEP, as your circumstances change.
Read about deciding to stop PrEP.
Changing your dosing schedule
PrEP can be individualised to your needs at different times.
If you are taking daily PrEP and want to switch to EBD, discuss this with your healthcare provider. They will help you decide if EBD is an option for you. They will make sure you change between daily and event based dosing safely.
If you are taking EBD and want to switch to daily dosing, start with a double dose. Take the medication at least 2 hours and no more than 24 hours before you have condomless sex. After that, continue taking PrEP daily.
If you switch from EBD to daily dosing, make sure that you have enough medication to take a dose every day before your next appointment with your PrEP service.
In some situations, your PrEP provider may suggest switching from daily to EBD to minimise the impact of the PrEP medication on your kidneys.
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 is limited information available 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 of HIV to the baby. So it may be appropriate to use PrEP in pregnancy. PrEP is recommended in pregnancy where there is a risk of HIV.
If you are taking PrEP and could become pregnant it is important to discuss this with your healthcare provider. This is to help you make an informed decision about what is best for you.
Drug resistance
If you get HIV while taking PrEP, there is a small risk of developing drug resistance to one or both drugs. This means that the drugs in PrEP (tenofovir and emtricitabine) will not work as well against HIV.
In PrEP studies, very few people got HIV while taking PrEP. In those who did, less than 1 in 20 developed drug resistance.
The possibility of drug resistance increases if you:
- start PrEP without knowing that you are already HIV positive - this is why it is very important to have a laboratory HIV blood test before you start PrEP
- take a break from PrEP and don’t check your HIV status before re-starting
- do not take PrEP correctly, get HIV and continue to take some PrEP medication
- come in contact with drug-resistant HIV. This is very rare - globally, only been 2 cases have been reported of PrEP not working where the person came in contact with drug-resistant HIV.
