If a person with HIV is taking antiretroviral treatment and has a fully suppressed viral load (‘undetectable’), the risk of HIV transmission through sharing sex toys is zero.
When HIV is not suppressed by antiretroviral treatment, the risk of HIV transmission through sharing sex toys (e.g. dildos, butt plugs and other objects meant to be inserted into the rectum and/or vagina) varies according to whether they are covered with condoms, and whether they are properly cleaned (i.e. disinfected) between use with different partners. Sex toys come into direct contact with rectal and/or vaginal mucosal membranes, which can both transmit HIV and are susceptible to infection. Mucosal membranes are the moist, inner linings of some organs and body cavities, such as the rectum and vagina. Sharing an unclean dildo or other toy can transfer infectious fluids from one person to another, making transmission possible.
As HIV doesn’t survive for long outside the body, the chances of transmission from a sex toy used days ago are minimal. If sex toys are used with new, clean condoms for each partner, or are properly cleaned (i.e. disinfected) before use by the HIV-negative partner, they pose a negligible risk of transmission. But using the same condom with both partners is effectively the same as not using one at all.
It is also safer to use sex toys that are made out of non-porous materials, such as body-safe silicone, ABS plastic, glass, stainless steel, or stone. (Sex toys that are porous are made out of materials like jelly, PVC or rubber.) Sex toys should be cleaned (i.e. disinfected) after each use to not only prevent transmission of HIV and STIs, but other complications such as bacteria and fungal infections.
There is zero HIV transmission risk attached to using sex toys if they are not shared. However, sex toys are often used before or after other sexual activities. As with fingering, fisting and other forms of play, any tissue damage or inflammation that results from sex toy use can affect the risk of infection during other activities.