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It is a hormonally active levorotatory enantiomer (l-norgestrel) of the racemic mixture
norgestrel.  It is a gonane progestin derived from 19-nortestosterone (1)


Oral contraception 
Used in monophasic and triphasic formulations
of both combined OCPs and progestogen only pill formulations.

Emergency contraception

Levonorgestrel can be administered in cases where emergency
contraception is required in either a single 1500 ug dose or
as 2 750 ug doses administered 12 hours apart.  This method
of "morning after" contraception is generally prescribed in cases
where unprotected intercourse has occurred with in the past
72 hours, however, some studied indicate it effectiveness
as late as 120 hrs (5 days) (2). 

The Tarascon Pocket Pharmacopoeia frequently carried
as a clinical reference indicates the Levonorgestrel (Plan B)
should be administered 1 tab PO "ASAP" but within 72
hr of intercourse, and the second tab 12 hr later.  The
kit commonly available contains 2 tabs .75 mg. 

Intrauterine contraceptions systems

Levonorgestrel is the active ingredient in the Mirena IUD.

Contraceptive Implants

Levonorgestrel is the active ingredient in the Norplant
and Jadelle contraceptive implants systems.

(1) Edgren RA, Stanczyk FZ (1999). "Nomenclature of the gonane progestins". Contraception 60 (6): 313.
(2) Trussell, James; Cleland, Kelly (2007-04-10). "Emergency Contraceptive Pills Worldwide"