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Oral Contraceptive Pills

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Last Update: February 17, 2019.


Currently, there are three types of oral contraceptive pills: combined estrogen-progesterone, progesterone only and the continuous or extended use pill.  The birth control pill is the most commonly prescribed form of contraception in the US.  Approximately 25% of women age 15-44 who currently use contraception reported using the pill as their method of choice.  The most commonly prescribed pill is the combined hormonal pill with estrogen and progesterone.  Progesterone is the hormone that prevents pregnancy, and the estrogen component will control menstrual bleeding.  Birth control pills are primarily used to prevent pregnancy.  The effectiveness of this form of birth control is referred to as typical and perfect use.  Typical use, meaning the method may not always be used consistently or correctly, results in a failure rate of 9 women out of 100 that will become pregnant during the first year of using this method.  Perfect use, meaning the method is used consistently and correctly every time, is less than one woman out of 100 will become pregnant in the first year of use.  Due to human error, the quoted failure rate for combined oral contraceptive pills is the typical use of 9%.  OCP’s can be used to address other health conditions particularly menstrual related disorders such as menstrual pain, irregular menstruation, fibroids, endometriosis-related pain and menstrual- related migraines.  Use of combined pills for acne has been formally approved by the FDA for specific brands.  The majority of women take OCP’s to prevent pregnancy, but 14% used them for non-contraceptive reasons.

Mechanism of Action

The progesterone is primarily responsible for preventing pregnancy.  The main mechanism of action is the prevention of ovulation; they inhibit follicular development and prevent ovulation.  Progestogen negative feedback works at the hypothalamus to decreases the pulse frequency of gonadotropin releasing hormone.  This, in turn, will decrease the secretion of follicle- stimulating hormone (FSH) and decreases the secretion of luteinizing hormone (LH).  If the follicle isn’t developing, then there is no increase in the estradiol levels (the follicle makes estradiol).  The progestogen negative feedback and lack of estrogen positive feedback on LH secretion stop the mid-cycle LH surge.  With no follicle developed and no LH surge to release the follicle, there is the prevention of ovulation.  Estrogen has some effect with inhibiting follicular development because of its negative feedback on the anterior pituitary with slows FSH secretion; it’s just not as prominent as the progesterone’s effect.  Another primary mechanism of action is progesterone’s ability to inhibit sperm from penetrating through the cervix and upper genital tract by making the cervical mucous unfriendly.  Progesterone induced endometrial atrophy should deter implantation, but there is no proof that this occurs. 


Combined oral contraceptive pills are to be taken daily at approximately the same time each day.  Avoid taking them greater than 24 hours apart as this could affect efficacy.  When you initiate the contraceptive pills you are not protected from pregnancy prevention in the first 7 days and an alternative method of birth control is recommended during this time period.  If you miss a tablet just take the missed tablet as soon as you remember and the next tablet at the usual time (taking 2 tablets in 1 day).  If you miss 2 tablets in a row in the first or second week then take 2 tablets the day you remember and 2 tablets the next day, then resume 1 per day.  Use another form of contraception until you begin a new cycle.   The extended cycle formulations have active hormone pills for every day for 3 months, followed by a week of placebo.  This can be manipulated and are formulations that involve taking active pill for 1 year which will functionally stop all menstrual bleeding; although the most common complication from extended cycle is break through bleeding.  Any formulation of combined oral contraceptive pill can be used in this manner but typically the monophasic pills the easiest to manipulate.  

Adverse Effects

Most side effects of OCP’s are mild and disappear with continued use or switching to another pill formulation.  The most common adverse effect of combined oral contraceptive pills is break through bleeding.  Women will also complain of nausea, headaches, abdominal cramping, breast tenderness, and an increase in vaginal discharge or decreased libido.  Nausea can be avoided by taking the medication at night before sleep.  The majority of the other consequences will resolve with time or switching OCP to a different preparation.  


There are a few absolute and relative contraindications of oral contraceptive use.   Women with uncontrolled hypertension should not initiate oral contraceptive use until their hypertension is being managed; diabetics may experience some hyperglycemia when initiating OC’s but these are issues that can be addressed.  However OCP’s are contraindicated in smokers over age 35 due to significant risk for cardiovascular events and specifically deep vein thromboembolism. Risk of venous thromboembolism is increased among OC users 3-9/ 10,000 woman-years compared with nonusers who are not pregnant and not taking hormones (1-5/10,000 woman-years), the risk is even greater in those over 35 and smoking.  Women with a history of VTE, known ischemic heart disease, migraines with auras, active or history of breast or endometrial cancer and valvular heart disease should not use OC’s as these conditions represent unacceptable health risks.  There are too many available alternatives for the patient to prevent pregnancy and not increase the patients’ health risk.  


Oral contraceptive pills provide patients with the option to prevent pregnancy.  If the patient has medical conditions that put them at increased risk for taking combined OC’s then there exist many alternatives to provide pregnancy prevention.  OC’s are a choice made by the patient and her physician after adequate counseling of risks and benefits.  There are significant noncontraceptive uses of hormonal contraceptives, and these should be considered when counseling the patient about her options.  Many OC formulations can provide menstrual regularity, treating both menorrhagia and dysmenorrhea.  They can even be utilized to induce amenorrhea for lifestyle considerations.  Strong epidemiologic evidence supports a 50% reduction in the risk of endometrial cancer among women who have used combined OCs compared with those who have never used combined OCs.  This effect lasts for up to 20 years.  Combined OC use decreases the risk of ovarian cancer by 27%, the longer the duration of use the greater the risk reduction.  OCs have also been reported to decrease the risk of colon cancer by 18%.  Some of the formulations even have indications for treatment of acne and hirsutism.  


If a patient takes too many oral contraceptive pills at one time the most likely complications will be severe headaches and nausea or vomiting.  There is no antidote to treat this condition, just treatment of the symptoms with antiemetics and analgesics.  If the patient has other risk factors significant for increased risk of venous thromboembolism one may consider using a prophylactic anticoagulant medication temporarily.  High doses of estrogen and progesterone (the same types found in the combined OC’s) are even treatment options for menorrhagia that has led to severe or symptomatic anemia.  


To access free multiple choice questions on this topic, click here.


Committee on Gynecologic Practice. ACOG Committee Opinion Number 540: Risk of venous thromboembolism among users of drospirenone-containing oral contraceptive pills. Obstet Gynecol. 2012 Nov;120(5):1239-42. [PubMed: 23090561]
ACOG Practice Bulletin No. 110: noncontraceptive uses of hormonal contraceptives. Obstet Gynecol. 2010 Jan;115(1):206-18. [PubMed: 20027071]
Maguire K, Westhoff C. The state of hormonal contraception today: established and emerging noncontraceptive health benefits. Am. J. Obstet. Gynecol. 2011 Oct;205(4 Suppl):S4-8. [PubMed: 21961824]
Baird DT, Glasier AF. Hormonal contraception. N. Engl. J. Med. 1993 May 27;328(21):1543-9. [PubMed: 8479492]
Shulman LP. The state of hormonal contraception today: benefits and risks of hormonal contraceptives: combined estrogen and progestin contraceptives. Am. J. Obstet. Gynecol. 2011 Oct;205(4 Suppl):S9-13. [PubMed: 21961825]
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Bookshelf ID: NBK430882PMID: 28613632


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