Glossary

TD (or tid) – a short hand term for medication dosing that means ‘take three times a day’.

See also q8H.

maternal-foetal microtransfusions are when small amounts of infected blood from the mother leak from the placenta to the baby during labour (or other disruption of the placenta).

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

DOT (directly observed therapy) – where a nurse or other healthcare worker observes or supervises someone take medicine.

formulation – the physical form that makes an active drug into a medicine.

Different formulations include

• Pills (capsules or tablets).
• Syrups or other oral solutions.
• Injections.
• Powders.
• Creams or lotions.

A co-formulation is when two active drugs are combined. For example: when two active drugs are in the same pill.