Plan B: Uses, Dosage, Side Effects, and Composition

Description and Composition of Plan B

Plan B is a hormonal drug that contain progesterone hormone called Levonorgestrel as its active pharmaceutical ingredient. In addition, it contains other inactive ingredients called excipients in sufficient quantities.

Tablets dosage Form

It comes in strength of 1.5 mg or 0.75 mg.

Levonorgestrel Intrauterine System

Plan B is also found in some IUDs. Levonorgestrel intrauterine system is a T-shaped plastic intrauterine device (IUD that is placed in the uterus where it slowly releases the hormone.
Levonorgestrel intrauterine system often contains 52 mg of levonorgestrel. Initially, levonorgestrel is released at a rate of approximately 20 mcg/day. This rate decreases progressively to approximately 10 mcg/day after 5 years and 9 mcg/day after 6 years

Indications and Uses of Plan B

Emergency Contraception (Tablets)

Plan B is used to prevent pregnancy after unprotected sex especially when one is not in her safe period or if one is few days to her ovulation. It should be used within 72 hours after unprotected sex for it to be effective.

Although, it can still be used even after 72 hours but not beyond 120 hours. It however works better if used early. It is estimated that Levonorgestrel will prevent 85% of expected pregnancies. 95% of expected pregnancies will be prevented if taken within the first 24 hours, declining to 58% if taken between 48 hours and 72 hours after unprotected intercourse.

Long-term contraception or nonemergency contraception (IUDs)

In addition to the above indication in emergency contraception, levonorgestrel is combined with other contraceptives in contraceptive formulations designed for regular use, for example with ethinyl estradiol. It is used in various hormone-releasing intrauterine devices for long-term contraception ranging for a duration of 3-5 years.

Hormone therapy and off-label uses

Levonorgestrel is prescribed in combination with estradiol as hormone therapy during menopause to manage vasomotor symptoms and to prevent osteoporosis. Off-label, levonorgestrel may be used to treat menorrhagia, endometrial hyperplasia, and endometriosis.

Dosage of Plan B 

Emergency Contraception (Tablets)

One tablet of Plan B 1.5mg to be taken once OR one tablet of Plan B 0.75mg to be taken and repeated after 12 hours.

Long-term contraception or nonemergency contraception (IUDs)

One IUD is inserted into your uterus by your doctor and removed at the manufacturer’s specified time. Levonorgestrel intrauterine system lasts between 3 to 5 years depending on brand. The 52-mg levonorgestrel-releasing intrauterine system is inserted in the uterus and releases up to 20 mcg of levonorgestrel per day (mean 14 mcg/day). The quantity of Levonorgestrel released per day progressively reduces over the years.

How Plan B works

Mechanism of action of Plan B

Plan B works in three different ways to prevent pregnancy.

  1. It prevents the release of eggs from the ovary (ovulation).
  2. It makes it difficult for sperm to get to an egg by thickening vaginal fluid.
  3. It prevents the attachment of a fertilised egg to womb.

Side effects of Plan B 

The side effects you may experience after taking/inserting Plan B include:

  • Vaginal bleeding
  • Breast pain or tenderness
  • Stomach pain
  • Headache
  • Dizziness
  • Tiredness
  • Nausea
  • Vomiting
  • Diarrhoea
  • Infections: for IUDs only

You may equally experience other side effects other than those listed above.

Pharmacokinetic properties of Plan B


Orally administered Plan B is rapidly and almost completely absorbed.

The absolute bioavailability of levonorgestrel was determined to be almost 100% of the dose administered.


Levonorgestrel is bound to serum albumin and sex hormone binding globulin(SHBG). Only about 1.5% of the total serum levels are present as free steroid. 65% are specifically bound to SHBG.

About 0.1% of the maternal dose can be transferred via milk to the nursed infant.


The biotransformation follows the known pathways of steroid metabolism, thelevonorgestrel is hydroxylated by liver enzymes mainly by CYP3A4 and itsvmetabolites are excreted after glucuronidation by liver glucuronidase enzymes.


After reaching maximum serum levels, the concentration of levonorgestrel decreased with a mean elimination half-life of about 26 hours.

Levonorgestrel is not excreted in unchanged form but as metabolites. Levonorgestrel metabolites are excreted in about equal proportions with urine and faeces.

Pharmacokinetics in obese women

A pharmacokinetic study showed that levonorgestrel concentrations are decreased in obese women (BMI ≥ 30 kg/m²) (approximately 50% decrease in Cmax and AUC0-24), compared to women with normal body mass index.

Is Plan B used in pregnancy?

Do not use in pregnancy

Can Plan B be used by a breast feeding mother?

Yes. It can be used. It is safe. However, about 0.1% of the maternal dose can be transferred via milk to the nursed infant.

Plan B Interactions with Drugs

Enzyme inducers, including CYP3A4, that metabolize progestins may decrease the plasma concentrations and the effectiveness of Plan B. These include:

  • barbiturates (including primidone)
  • bosentan
  • carbamazepine
  • felbamate
  • griseofulvin
  • oxcarbazepine
  • phenytoin
  • rifampin
  • St. John’s wort
  • topiramate
  • Efavirenz
  •  Protease inhibitors (increase or decrease)
  •  Non-nucleoside reverse transcriptase inhibitors (increase or decrease)


This write up is neither a prescription, medical advice nor a substitute of any of them. You are therefore advised to strictly rely and  adhere to your physician’s prescription.
Remember not to take any drug without consulting your health care provider first.
Self medication can be very deadly.


  1. Drug Bank
  2. emc
  3. NCBI
  4. Journal: Obsertrical & Gynecological Survey
  5. Levonorgestrel Information
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