Glossary

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.

• Event-based oral PrEP dosing
• Examples of event-based 2:1:1 and 2:7 dosing

placenta – the placenta is a temporary organ that develops in pregnancy and joins the mother and foetus. The placenta acts as a filter. It transfers oxygen and nutrients from the mother to the foetus, and takes away carbon dioxide and waste products. The placenta is full of blood vessels.

The placenta is expelled from the mother’s body after the baby is born and it is no longer needed. It is sometimes called the afterbirth.

foetal membranes are the membranes surrounding the foetus.

primary mutation – see major mutation.

AIDS – acquired immune deficiency syndrome.