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 sex and for several days afterwards.
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 and how often you have sex:
- daily PrEP (taking PrEP every day)
- event based dosing (taking PrEP around the time of sex)
Daily PrEP: for anal and vaginal/frontal sex
Take 1 pill per day, every day.
Taking PrEP every day will make sure that there are protective drug levels in vaginal and anal tissue, 24 hours a day, 7 days a week.
This means you do not have to plan when to have sex. For people who usually have condomless sex at least once a week, daily PrEP is likely to be a better dosing option.
You should try to take PrEP at the same time every day. It does not have to be the exact same time but it will help you get into a routine.
PrEP and anal sex
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 anal sex.
Continue taking 1 tablet of PrEP per day, every day.
If you miss a PrEP dose
Daily PrEP allows some flexibility for anal sex. Once you are established on daily PrEP, if you occasionally miss 1 or 2 doses, continue taking it when you remember. Do not stop taking PrEP.
For anal sex, you must at the very least have taken PrEP for 4 days a week to have good protection. If you miss more doses than this, you are not going to be protected against HIV if you are having condomless sex.
If you have missed more than 3 doses of PrEP in the week and had condomless sex, you may need PEP (post exposure prophylaxis). You should discuss this with your healthcare provider as soon as possible and within 72 hours of having condomless sex.
If you are missing doses regularly, please discuss this with your healthcare provider. They will give you advice on what to do.
PrEP and vaginal/frontal sex
For vaginal/frontal sex, you need to take daily PrEP. Event based dosing is not suitable.
You need to take PrEP every day because PrEP does not get into the vaginal tissues as quickly as it gets into rectal tissues.
When you start taking PrEP, you need to take it every day for 7 days before having condomless sex.
If you miss a PrEP dose
There is less flexibility around missing doses for vaginal/frontal sex. It is important to establish a routine for taking the medication.
For vaginal/frontal sex, you must at the very least have taken PrEP for 6 days a week to have good protection. If you miss more than 1 dose a week, you are not going to be protected against HIV if you are having condomless sex.
If you have missed more than 1 dose of PrEP in the week and had condomless sex, you may need PEP. You should discuss this with your healthcare provider as soon as possible and within 72 hours of having condomless sex.
If you are regularly missing doses, discuss this with your healthcare provider.
Tips on how to take 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): only for anal sex
Event based dosing (EBD) should only be used in certain situations. 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
EBD is suitable only for anal sex.
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:
- for vaginal/frontal sex
- if you have hepatitis B - it is very important to know your hepatitis B status before taking PrEP
Event based dosing for sex once in a week
- 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 the body when you have sex.
- Take a single pill 24 hours after the double dose.
- Take another single pill the following day, 24 hours later.
Event based dosing for sex several times over a period of time
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) as usual at least 2 (but no more than 24 hours) before sex.
- If you are continuing to have sex, take 1 pill 24 hours after the double dose and continue taking 1 pill 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 to make sure you understand how to take EBD correctly before considering EBD for PrEP. Your healthcare provider can explain this to you and answer any questions you may have.
If you miss a PrEP dose
It is important not to miss doses if you are taking event based PrEP.
In a situation where you have missed doses of PrEP and haven’t used a condom or had a condom accident, you may need PEP.
If you missed the before dose completely, still take a double dose as soon as possible after sex. Continue this daily, but contact your clinic as soon as possible and within 72 hours. Depending on the risk involved, you may need to go on PEP which will include an additional HIV drug.
Changing how you take PrEP
Your risk of HIV may change over time. You can change how you take PrEP or stop and restart PrEP, as your circumstances change.
Read about stopping PrEP.
Changing your dosing schedule
PrEP can be individualised to your needs at different times.
If you are taking 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.
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.
Becoming HIV positive during pregnancy is associated with an increased risk of transmission of HIV to the baby. So it may be appropriate to use PrEP in pregnancy.
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.
If you become HIV positive 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 became HIV positive 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 and become HIV positive
- 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.