selective pressure – this is when a factor in the environment causes one type of organism to develop and grow in preference to another. With HIV drug resistance, the presence of a drug exerts selective pressure for resistance to develop. It is based on evolution and the concept of ‘survival of the fittest’ theory developed by Charles Darwin.
Glossary
Selected words and phrases
study population – the group of people in a study.
genotype (or genotypic) – This term has several common uses:
1. a general term for the molecular structure of a living organism or virus.
2. a test that looks at the structure of an organism of virus (ie genotype resistance test looks for changes in the virus structure – called mutations).
3. a category for different types of similar viruses – ie hepatitis C has many subtypes, referred to as genotype-1, genotype-2 etc. HCV genotype is the strongest predictor of response to hepatitis C treatment.
See also phenotype.
malignant – dangerous. A malignant cancer is a dangerous cancer that is growing.
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.