PrEP is not considered one hundred percent effective primarily because its real-world success relies heavily on human behavior and consistent medication adherence. If a user frequently misses doses, the concentration of the drug in their body drops below the threshold required to effectively block the virus. While clinical trials show near-perfect protection when taken precisely as prescribed, perfect compliance is difficult to maintain consistently over long periods. Additionally, extremely rare instances of drug-resistant strains of HIV or inconsistent timing relative to exposure events can occasionally impact overall protective efficacy.
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