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Glossary

Selected words and phrases

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.

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.