syphilis – an STI that is a bacterial infection, usually transmitted by sexual contact. Untreated, syphilis can cause serious injury to an unborn baby.
Glossary
Selected words and phrases
On-demand dosing – a term for taking medicines only at times linked to a relevant event.
On-demand dosing for PrEP is also called “event-based dosing” (EBD).
For example, on-demand dosing for PrEP (to prevent HIV transmission) involves taking PrEP before sex and then for several days after sex.
For many years, on-demand dosing referred to 2:1:1 dosing. This was an option for cisgender gay and bisexual men using PrEP. This involves starting with a double dose (two pills), taken 24 to 2 hours before sex, and then single daily doses for the two days after sex.
In 2025, UK (BASHH) and European (EACS) PrEP guidelines included 2:7 on-demand dosing for cisgender women and for transgender and non-binary people.
replicate – to duplicate, copy or reproduce. It is more scientifically accurate to refer to a virus replicating than reproducing.
adverse event – an unexpected or unwanted effect during a research study. Sometimes this used to describe a side effect from a drug.
Although all side effects are adverse events, not all adverse events are side effects.
PI (protease inhibitor) – a type of HIV drug.
HIV protease inhibitors include atazanavir, darunavir, fosamprenavir, indinavir, nelfinavir, lopinavir/r (Kaletra), ritonavir, saquinavir and tipranavir.
Drug targets in the virus life cycle
Different protease inhibitors have been developed to treat hepatitis C.