A person's hand holding a blue tablet labeled "PEP" against a dark, satin fabric background, with a black and gold marble box in the corner, promoting post-exposure prophylaxis for HIV prevention.

PEP (Post-Exposure Prophylaxis)

PEP (Post-Exposure Prophylaxis) is an emergency HIV prevention treatment used after a possible HIV exposure to help prevent infection before the virus establishes itself in the body.

Unlike PrEP, which is taken proactively before exposure, PEP is used after an unexpected risk event.

PEP is considered a medical urgency—not a treatment to delay.

The sooner treatment is started, the better the chance of success.

When Should PEP Be Started?

PEP must be started within 72 hours (3 days) of a potential HIV exposure.

However, clinical guidance is clear:

Earlier is significantly better.
Ideally, treatment should begin within the first 24 hours whenever possible.

If more than 72 hours have passed, PEP is generally no longer recommended because effectiveness decreases substantially.

How Effective Is PEP?

When prescribed appropriately, started promptly, and taken exactly as directed for the full course, PEP is highly effective at reducing the risk of HIV infection.

Because randomized placebo-controlled human trials would be unethical, exact percentages vary depending on study design and exposure circumstances. Historical occupational data demonstrated approximately 81% risk reduction, while modern combination regimens are believed to be substantially more effective when adherence is optimal.

Important realities:

  • PEP is not 100% effective

  • Missing doses reduces protection

  • Delayed initiation reduces effectiveness

  • Repeated ongoing exposures may require a different prevention strategy (PrEP)

Situations Where PEP May Be Appropriate

PEP may be considered after:

  • Condom failure during vaginal or anal sex

  • Unprotected sex with a partner of unknown HIV status

  • Exposure involving a partner known to have HIV who is not confirmed undetectable

  • Shared needles or injection equipment

  • Sexual assault

  • Occupational exposure (such as needlestick injuries)

PEP is generally not necessary when:

  • More than 72 hours have passed

  • The exposure carries negligible HIV risk

  • The source partner is confirmed undetectable (U=U)

What Testing Is Needed?

Before starting PEP, baseline testing is recommended whenever feasible—but treatment should not be delayed if PEP is clearly indicated.

Baseline Evaluation May Include:

HIV Testing

  • Rapid 4th generation HIV test preferred

Kidney Function

  • Creatinine / renal function assessment

Liver Function

  • Hepatic enzyme evaluation

STI Screening

  • Gonorrhea

  • Chlamydia

  • Syphilis

Hepatitis Testing

  • Hepatitis B

  • Hepatitis C

Pregnancy Test

  • When clinically appropriate

Follow-Up Monitoring

PEP requires follow-up to confirm effectiveness and monitor safety.

Typical follow-up includes:

During / Shortly After Treatment

  • symptom review

  • medication tolerance assessment

  • adherence counseling

HIV Retesting

Common monitoring timeline:

  • baseline

  • 4–6 weeks

  • 3 months after exposure

Additional testing may be individualized depending on exposure details or special circumstances.