preclinical refers to the early development stage for a new drug, before any testing on humans. This includes they laboratory and animal tests. Preclinical studies will not always show what will happen when people use the drugs, but can provide a guide to serious problems that could occur.
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 cis and trans women and other transgender and non-binary people.
mg (milligram) – abbreviation for milligram. 1000 milligrams = 1 gram.
half-life (T1/2) the time taken a drug to clear from the highest concentration to half this level. Drugs have different half-lives in different compartments (ie half-life in blood can be different from the half-life inside a cell). It take 5 x the half-live for a drug to be considered cleared.
log value – a number expressed as a factor of 10 – ie 2 logs = 10 x 10=100; 3 logs = 10 x 10 x 10=1000. This measurement is used when needing to include very large and very small numbers on the same scale. A reduction by one log is a 90% reduction. A reduction by two logs is a 99% reduction etc.