This Date in UCSF History: Plan B vs. RU486

Tuesday, March 7, 2023

Originally published in Synapse on March 8, 2007.

As contraception and abortion remain hot topics in the news, there is much discussion of both Plan B and RU4B6. Disappointingly, there is a great deal of inaccurate information portrayed in the media, a major source of health information for many. As healthcare professionals, we have a duty to clear any confusion regarding these drugs to our patients and the general public. And in order to do that, we must be informed about the drugs ourselves.

The major difference between the two drugs is that Plan B is a contraceptive agent, whereas RU4B6 is an abortifacient. This means that Plan B does not interfere with a previously established pregnancy. This is a key distinction that many are not aware of. Beyond realizing this distinguishing fact, it is important to be aware of the use, mechanism of action and side effects of each drug.

Plan B (levonogestrel) contains two tablets, each containing 0.75mg of levonogestrel, a progestin. This drug contains a higher dose of the same progestins component found in many oral contraceptive pills.

The mechanism of action of this drug is to prevent ovulation. There are other proposed mechanisms of inhibiting fertilization or implantation, which are not well studied. This product can be used at any time in the menstrual cycle.

Plan B should be initiated within 72 hours of unprotected sex to achieve an efficacy of 89%. The sooner it’s taken, the more likely it is to work. It is also recommended to initiate therapy within 120 hours of unprotected sex to reduce the risk of pregnancy by 75%.

Clearly, emergency contraception is not as effective as routine contraception. There are two ways to take the full dose. The first tablet can be taken, followed by the second tablet in 12 hours. Alternatively, both tablets can be taken together at once.

In recommending one regimen over the other, the provider should consider the patient’s history of nausea and vomiting, as well as adherence to drug regimens. Approximately 23% of patients will experience nausea and 6% of patients will experience vomiting after taking Plan B. If a patient vomits within one hour of taking either tablet, a repeat dose may be necessary to ensure efficacy.

Also, an antiemetic such as meclizine can be taken 30 minutes prior to the dose to reduce any anticipated nausea and vomiting. Other common side effects include abdominal pain, fatigue and headache. Some patients may see some spotting a few days after taking Plan B and the majority will have their menstrual period within a week of the anticipated start with the same menstrual flow as usual.

In August 2006, Plan B was approved for over-the-counter availability for patients aged 18 and older, in addition to its prescription availability for all women. Although Plan B is available for adult women over-the-counter, it can only be found in licensed pharmacies and healthcare clinics. Most likely, your patients will find it “behind-the-counter” at the pharmacy.

RU4B6, also known as mifepristone or Mifeprex, is the only agent currently available for medical abortions.

The drug can be taken orally within the first 49 days of pregnancy to induce an abortion. Mifepristone is a competitive inhibitor of progesterone. The dose is 600mg of mifepristone, often followed by 400mcg of misoprostol in 48 hours. Misoprostol is a prostaglandin analog that is used to induce myometrial contractions and expulsion.

Administration of mifepristone with the misoprostol has shown a 96% to 98% abortion rate. Side effects include abnormal vaginal bleeding (90%), cramps (90%), nausea (61%), vomiting (26%) and diarrhea (20%). Mifepristone can only be administered in a medical office, clinic or hospital where there is supervision by a physician who is able to assess the gestational age of an embryo and rule out ectopic pregnancy.

Hopefully, this overview has been helpful in differentiating between Plan B and RU4B6, as well as reviewing key points.