Quoting from the red cross FAQ:

The waiting period is required due to these drugs interfering with viral replication and thus possibly altering the detectability of diagnostic and screening tests for HIV, including extending the window period prior to detectable infection or a delay in producing antibodies.

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The Red Cross and the FDA support individuals making responsible choices for their health and the broader health of our communities. The Red Cross and FDA are involved in ongoing research, data collection and assessment related to transfusion safety, including the use of HIV preventative medications, and will continue to seek opportunities that could potentially help lead to additional changes. The Red Cross does not encourage individuals to stop taking these medications in order to donate blood.