Although PrEP is much more expensive than most other HIV prevention methods, studies suggest that it may be cost-effective in some circumstances. Moreover, it may sometimes be cost-saving, in other words costing less to prevent one HIV infection than the lifetime healthcare costs of that infection.

A key determinant of cost-effectiveness is selective use – PrEP is only cost-effective if it is prioritised for people at very high risk of HIV. A UK analysis found that PrEP could be cost-effective and even cost-saving if it was provided to gay men who have recently had a sexually transmitted infection or condomless sex with at least five casual partners in the past three months. However, PrEP would not be cost-effective if used by people at lower risk of HIV infection. The cost of providing PrEP to larger numbers of people would not be offset by a significant reduction in new infections.

A second determinant is the cost of the drugs used, showing the importance of using generic drugs and of pharmaceutical companies lowering their prices. The same UK analysis found that if drug prices were halved, PrEP could still be cost-effective even if it were offered to a wider range of gay men (for example, any man reporting condomless sex with a casual partner).

For the UK, there are no relevant analyses of the cost-effectiveness of PrEP in groups other than gay men.

Whether the NHS will consider PrEP to be affordable is a different question. Making PrEP widely available would be quite expensive in the short term and the financial benefits it should bring (reductions in spending on antiretroviral drugs) will not be seen for several decades.

Truvada is a fixed-dose combination which contains two drugs, tenofovir and emtricitabine. At present, a year’s supply officially costs £4330 per person, although the NHS obtains a discount and may pay around £3000 per year.

The cost of PrEP will change substantially when the drugs are no longer protected by patent. Tenofovir will come off patent in late 2017 and emtricitabine may lose its patent protection sometime between 2017 and 2021. This means that the pharmaceutical company which originally developed the drugs will no longer have the exclusive right to manufacture them. Rival companies will be able to produce cheaper versions, which could be up to 80% less expensive.

The annual cost of healthcare, including antiretroviral drugs, for a person living with HIV is around double that of the annual cost of providing PrEP. People need to take HIV treatment for the rest of their life but PrEP will generally be needed for a much shorter period.