Mechanisms of action: The pill, the implant, the shot, and IUDs
Details and research on the ways each hormonal contraceptive method prevents pregnancy
Contents
Introduction
Hormonal contraceptives (HCs) are a method of contraception that acts on the endocrine system (the body's “messenger” system) through the use of hormones to prevent pregnancy.1 Hormonal contraceptives may contain both estrogen and progestin, or may only contain progestin.2
The way a contraceptive works is called its mechanism of action.
There are a variety of hormonal contraceptive methods: pills, intrauterine devices (IUDs), patches, injections, vaginal rings, and implants. Different methods of hormonal contraception can have differing mechanisms of action.
Below is a quick guide to the mechanism of action for each method of hormonal contraception. 👇
🟢 Contraceptive Implant
The only hormonal implant currently available in the United States is Nexplanon®, “a single-rod system releasing etonogestrel (ENG),” a type of progestin.3 Nexplanon® “is indicated by the FDA to provide contraception for up to 3 years.”4
Mechanism of Action: The Nexplanon® implant “prevents pregnancy through inhibition of ovulation and blocking fertilization through cervical mucus thickening.”5
Etonogestrel (ENG) “concentrations of at least 90 pg/mL [is] sufficient for ovulation suppression.”6 “To determine how long ovulation was reliably suppressed, investigators evaluated follicular development, endometrial thickness, and serum concentrations of 17 b-estradiol and progesterone twice weekly for 3 years. They found that the ENG implant universally suppressed ovulation for 30 months,” though there can be “interindividual differences in serum levels.”7
🟢 Hormonal Intrauterine Devices (IUDs)
“An IUD is a type of long-acting, reversible contraception (LARC).”8 “A small, flexible, T-shaped device, an IUD is inserted into the uterus through the vagina.”9 Hormonal IUDs contain a progestin called levonorgestrel (LNG). “Currently, there are four LNG IUDs available in the United States. All have T-shaped frames with a reservoir containing levonorgestrel dispersed in polydimethylsiloxane molded around the vertical stem. The reservoirs are covered with a silicone membrane that regulates the release of the hormone. The stems have monofilament polyethylene threads attached to the base to assist with removal.”10
Mechanism of Action: Intrauterine devices (IUDs) “work primarily by preventing sperm from fertilizing ova.”11 Fertilization is prevented by the release of levonorgestrel.12 “The progestin in LNG IUDs thickens cervical mucus, suppresses the endometrium by reducing uterine artery blood flow, and impairs sperm function by changing the uterine immune microenvironment. In addition, ovulation may be impaired because of systemic absorption of levonorgestrel.”13
It has been theorized that hormonal IUDs also prevent pregnancy through a "foreign body effect" due to the presence of the IUD's plastic frame being inserted into the uterus.14 However, more recent research “argue against a foreign body reaction as a common mechanism of action of IUDs.”1516
🟢 Injectable Contraceptives
“The most commonly used injectable contraceptives contain depot medroxyprogesterone acetate (DMPA),” a type of progestin, and is “marketed as Depo-ProveraⓇ.”17 DMPA “is structurally related to natural progesterone.”18
Mechanism of Action: “The primary contraceptive mechanism of action of DMPA is inhibition of ovulation.”19
“Ovulation inhibition occurs through a series of positive and negative feedback loops resulting in the suppression of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release from the pituitary gland, thus suppressing the mid-cycle LH surge.”20
“Ovulation inhibition provided by DMPA is substantial: In a study measuring estradiol and progesterone levels, and follicular development per ultrasound in new DMPA users, no ovulations occurred when DMPA was administered by day 9 of the menstrual cycle.”21
“DMPA also has a significant and rapid effect on the cervical mucus, effectively thickening the cervical mucus to prevent sperm transport. In a study measuring cervical mucus scores in new DMPA users, 90% of the participants had a poor cervical mucus score by 24 hours after injection.”22
🟢 Combined Oral Contraceptives (COCs)
Combined oral contraceptives (COCs), otherwise known as birth control pills, “are methods that contain both an estrogen and a progestin component.”23 The estrogen component is “most often ethinyl estradiol (EE).”24
Mechanism of Action: “COCs prevent fertilization.”25
“The progestin component provides the main contraceptive effect through ovulation suppression.”26 The ethinyl estradiol (EE) estrogen component “makes a small contribution to ovulation suppression but primarily helps with cycle control.”27
“Ovulation suppression occurs through a series of positive and negative feedback loops resulting in pituitary suppression, thus suppressing the luteinizing hormone (LH) surge (progestin effect) and release of follicle-stimulating hormone (FSH; estrogen effect).”28 “Ovulation suppression provided by even low-dose [combined oral contraceptives] formulations is profound: they suppress 99% to 100% of cyclic ovulations as reported in clinical trials that detected ovulation through serum hormone monitoring.”29
🟢 Progestin-Only Pills (POPs)
Progestin-only birth control pills, which are sometimes called minipills, contain a progestin hormone, but no estrogen component.30 “POPs can be a highly effective contraceptive method when taken as directed. They are included in the second tier of effectiveness, along with COC pills, injectable contraceptives, and hormonal patches and rings.”31 “Progestin-only pills are safe for almost all people, including many with contraindications to combined oral contraceptive pills.”32 “The most common side effect of progestin-only pill users is irregular bleeding.”33
Mechanism of Action: POPs prevent pregnancy through the following mechanisms.
“Thickens cervical mucus, which impairs sperm penetration.”34
“Alters luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion, resulting in inhibition of ovulation in some proportion of cycles.”35
“Decreases tubal motility by reducing activity of the cilia in the fallopian tube, which prevents the sperm and the egg from meeting.”36
“The specific mechanism active in a particular cycle may vary among users and, in any single user, between cycles. Studies found drospirenone 4 mg inhibits ovulation as effectively as desogestrel, which has ovulation rates similar to combined hormonal contraceptives.”37
🟢 Emergency Contraceptive Pills (EC)
“Emergency contraception refers to methods of contraception that can be used to prevent pregnancy after sexual intercourse. These are recommended for use within 5 days but are more effective the sooner they are used after the act of intercourse.”38 EC prevents pregnancy from occurring. EC must be used soon after unprotected sexual intercourse to be effective. It does not work if pregnancy has already occurred.”39 “The morning after pill only works by preventing ovulation, it can not cause an abortion and it will not have any effect in a case where an egg has already been fertilized.”40
Mechanism of Action: Emergency contraceptive (EC) pills prevent pregnancy by preventing or delaying ovulation.41 Emergency contraception pills, “disrupt the feedback system between the brain and ovaries, thus inhibiting the release of an egg.”42
To learn more about emergency contraception, click HERE.
NCI Dictionary of Cancer terms. Comprehensive Cancer Information - NCI. (n.d.). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/hormonal-contraception
NCI Dictionary of Cancer terms. Comprehensive Cancer Information - NCI. (n.d.). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/hormonal-contraception
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MacMillan, C. (2023, October 4). Intrauterine devices (iuds): What women need to know. Yale Medicine. https://www.yalemedicine.org/news/intrauterine-devices-iud
MacMillan, C. (2023, October 4). Intrauterine devices (iuds): What women need to know. Yale Medicine. https://www.yalemedicine.org/news/intrauterine-devices-iud
Economou, N., Mengesha, B., & Schwarz, E. B. (2025). Intrauterine Devices (IUDs). In Contraceptive Technology (22nd ed., p. 343). essay, Jones & Bartlett Learning.
Economou, N., Mengesha, B., & Schwarz, E. B. (2025). Intrauterine Devices (IUDs). In Contraceptive Technology (22nd ed., p. 344). essay, Jones & Bartlett Learning.
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Smith-McCune, K., Thomas, R., Averbach, S., Seidman, D., Takeda, M., Houshdaran, S., & Giudice, L. C. (2020, April 23). Differential effects of the hormonal and copper intrauterine device on the endometrial transcriptome. Nature News. https://www.nature.com/articles/s41598-020-63798-8
Study: Copper iuds do not appear to prevent implantation or increase... Relias Media. (2020, July 1). https://www.reliasmedia.com/articles/146320-study-copper-iuds-do-not-appear-to-prevent-implantation-or-increase-hiv-risk
Lawley, M., & Cwiak, C. (2025). Injectable Contraceptives. In Contraceptive Technology (22nd ed., p. 377). essay, Jones & Bartlett Learning.
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American College of Obstetricians and Gynecologists. (2019). Emergency contraception. ACOG. https://www.acog.org/womens-health/faqs/emergency-contraception
The ella® pill: All you need to know about ella®: Ella®. ellaone USA. (2020, July 20). https://www.ella-now.com/all-you-need-to-know-about-taking-ella/#:~:text=By%20taking%20ella%C2%AE%20as,egg%20has%20already%20been%20fertilised
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