It is well established that preexposure prophylaxis with Truvada is effective to prevent HIV infection when the individual is 100% adherent to medication.
However, there is not much information about potential failure and contracting this infection in spite of high adherence in a case of viral resistance.
A case from Toronto was recently reported, when a 43 y/o man contracted HIV infection in spite of beeing highly adherent to PrEP for two years.
The HIV virus he contracted was resistant to Truvada (Emtriva and Tenofovir).
This illustrates that preexposure prophylaxis is not perfect, and may not be 100% effective, therefore condom use should be part of prevention of sexual transmission of HIV infection.