obstetrics – a branch of medicine that refers to the care of women during pregnancy.
Glossary
O
oesophagus – part of the gut between the throat and the stomach.
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.
open label – this means that a patient in a trial knows which treatment they are taking.
opiate – a drug that comes from the opium poppy. Opiates include heroin, codeine and morphine.
opportunistic infection (OI) – an infection that occurs after your immune system has been damaged by HIV.
organogenesis – the process of development of baby’s main organs. This period is the first 12 weeks in the womb.
outreach – a type of service which goes out to find and engage people.