Written on August 13, 2023 by Gillian Singer, MPH, Sexuality Educator & Certified Sexologist. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.
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You may be wondering about the similarities and differences between PrEP vs PEP. Both are HIV prevention methods, but what is the difference?
HIV is a sexually transmitted infection (STI) that can be spread by contact with infected blood or from illicit injection drug use or sharing needles. It can also be spread from mother to child during pregnancy, childbirth, or breastfeeding.
If a person is HIV-positive and is undergoing consistent antiretroviral therapy (ART) as prescribed by a healthcare provider, they can achieve an undetectable viral load of HIV. If the virus is undetectable, you cannot transmit the virus to anyone who is HIV-negative sexually.
PrEP stands for pre-exposure prophylaxis and is an antiretroviral medication that works to prevent the contraction of Human Immunodeficiency Virus (HIV). According to the CDC, PrEP can reduce the odds of contracting HIV from sex or injection drug use. When taken as prescribed, PrEP is highly effective for preventing HIV.
PrEP, which is a pre-exposure strategy to reduce the risk of HIV transmission, is often taken by people who are at a higher risk for contracting HIV — for example, people who have sexual partners who are HIV positive or have multiple or many partners whose HIV status is unknown. Other high-risk groups PrEP is typically recommended for include, those with an HIV-positive sexual partner, individuals engaging in unprotected sex, people who inject drugs, and other high-risk populations. PrEP is only taken by people who are HIV-negative.
PrEP, when used correctly and as prescribed, can reduce the risk of getting HIV by around 99%. It is known that PrEP can reduce the risk of getting HIV by at least 74% for people who inject drugs.
PrEP comes in the form of pills or shots:
Both Truvada® (emtricitabine and tenofovir disoproxil fumarate) and Descovy® (emtricitabine and tenofovir alafenamide) are taken daily and require one to be vigilant about taking their medication.[4,5]
Apretude® (cabotegravir) is administered bi-monthly by a healthcare provider after initial injections have been given 1 month apart for 2 consecutive months. You should stay under a provider’s care while receiving Apretude® and must receive it as scheduled. Call your provider right away if you will miss a scheduled injection by more than 7 days.
Apretude® can cause fever, general feelings of illness, tiredness, muscle or joint aches, trouble breathing, blisters or sores in the mouth, redness or swelling of the eyes, swelling of the mouth, face, lips, or tongue, and/or liver problems.
Truvada® (as a form of PrEP) can cause headaches, pain in the abdomen, and/or weight loss.
Descovy® may cause diarrhea, nausea, headache, fatigue, and/or stomach discomfort.
PrEP pills reach their maximum effect at about 7 days of daily use for receptive anal sex (also known as bottoming). PrEP reaches its maximum effect after 21 days of daily use for receptive vaginal sex and injection drug use. It is unknown how long it takes to be most effective to protect those who are participating in insertive anal sex (topping) or insertive vaginal sex.
PrEP works by preventing HIV from replicating in the body. It acts as a catalyst that helps the body produce antibodies, which help diseases causing germs and viruses. After contact with the virus, the tenofovir and emtricitabine block the enzyme needed by the virus to replicate. If used correctly, PrEP eliminates the risk of contracting the virus once exposed to it. 
Unlike PrEP, Post-exposure prophylaxis (PEP) is used after potential exposure to HIV to prevent the virus from establishing itself. PEP must be taken within 72 hours of the potential or known exposure to the virus and the combination of antiretroviral medications must be taken for 28 days.
Those who should consider taking PEP include people who experience a contraceptive error or failure (for example, a broken condom), people who share injectable drug paraphernalia, and those who are sexually assaulted.
PEP is intended for emergency situations, such as when someone has had unprotected sex with a partner whose HIV status is unknown or who is known to be HIV-positive without appropriate viral suppression, or when someone has experienced a needlestick injury with a potentially HIV-contaminated needle. It is not a guarantee of preventing HIV infection, but when initiated promptly after potential exposure, it can significantly reduce the risk of transmission.
PEP can help prevent the virus from replicating after recent exposure. Cells originally infected with HIV die naturally within a short period of time, reducing the likelihood for HIV to establish itself in the body.
PEP may cause nausea, upset stomach, fatigue, and headaches, but symptoms often go away within a week of starting the medication.
To find out your HIV status, you can test for HIV in the privacy of your own home with an at-home HIV test through Everlywell. A test only requires a small blood sample via a simple finger stick. Your results can be viewed through Everlywell’s secure online platform. If your test returns positive, a healthcare provider will connect with you to discuss your case and the appropriate next steps.
As explained above, PrEP and PEP are both preventative measures to prevent the transmission and contraction of HIV. PrEP is taken prior to exposure to HIV, whereas PEP is taken after potential exposure.