Pharmacotherapeutic group: Sex hormones and modulators of the genital system, emergency contraceptives, ATC code: G03AD01
Mechanism of action
. At the recommended regimen, levonorgestrel is thought to work mainly by preventing ovulation and fertilisation if intercourse has taken place in the preovulatory phase, when the likelihood of fertilisation is the highest. Levonorgestrel is not effective once the process of implantation has begun.
Clinical efficacy and safety
Results from the randomised, double-blind clinical studies conducted in 1998, 2001and 2010 showed that a 1500 microgram levonorgestrel (taken within 72 hours of unprotected sex) prevented 85%, 84%, 97% of expected pregnancies respectively.
The pregnancy rate (number of observed pregnancies in women taking EC/total number of women taking EC) was 1.1%, 1.34%, and 0.32%, respectively. Prevented fraction appeared to decrease and pregnancy rates appeared to increase with time of start of treatment after unprotected intercourse, highest efficacy is reached when EC is taken within 24 hours after intercourse. Efficacy appears to decrease with increasing time from unprotected intercourse.
Meta-analysis on three WHO studies (Von Hertzen et al., 1998 and 2002; Dada et al., 2010) showed that the pregnancy rate of levonorgestrel is 1.01% (59/5 863) (compared to an expected pregnancy rate of about 8% in the absence of emergency contraception) see Table 1.
Table 1: Meta-analysis on three WHO studies (Von Hertzen et al., 1998 and 2002; Dada et al., 2010)
| Levonorgestrel dose | Treatment delay in days | Prevented fraction (95% CI)* | Pregnancy rate |
Von Hertzen, 1998 | 0.75 mg (two doses taken 12 h apart) | Day 1 (≤ 24 h) | 95% | 0.4% |
Day 2 (25-48 h) | 85% | 1.2% |
Day 3 (49-72 h) | 58% | 2.7% |
All women | 85% | 1.1% |
Von Hertzen, 2002 | 1.5 mg (single dose) | 1-3 days | 84% | 1.34% |
0.75 mg (two doses taken together) | 1-3 days | 79% | 1.69% |
Dada, 2010 | 1.5 mg (single dose) | 1-3 days | 96.7% | 0.40% |
0.75 mg (two doses taken together) | 1-3 days | 97.4% | 0.32% |
Meta-analysis of all three WHO studies | - | - | 1.01% |
*CI: confidence interval (compared to an expected pregnancy rate of about 8% in the absence of emergency contraception)
There is limited and inconclusive data on the effect of high body weight/high BMI on the contraceptive efficacy. In three WHO studies no trend for a reduced efficacy with increasing body weight/BMI was observed (Table 2), whereas in the two other studies (Creinin et al., 2006 and Glasier et al., 2010) a reduced contraceptive efficacy was observed with increasing body weight or BMI (Table 2). Both meta-analyses excluded intake later than 72 hours after unprotected intercourse (i.e. off-label use of levonorgestrel) and women who had further acts of unprotected intercourse. (For pharmacokinetic studies in obese women see section 5.2).
Table 2: Meta-analysis on three WHO studies (Von Hertzen et al., 1998 and 2002; Dada et al., 2010)
BMI (kg/m2) | Underweight 0-18.5 | Normal 18.5-25 | Overweight 25-30 | Obese ≥ 30 |
N total | 600 | 3952 | 1051 | 256 |
N pregnancies | 11 | 39 | 6 | 3 |
Pregnancy rate | 1.83% | 0.99% | 0.57% | 1.17% |
Confidence Interval | 0.92 3.26 | 0.70-1.35 | 0.21-1.24 | 0.24-3.39 |
Table 3: Meta-analysis on studies of Creinin et al., 2006 and Glasier et al., 2010
BMI (kg/m2) | Underweight 0-18.5 | Normal 18.5-25 | Overweight 25-30 | Obese ≥ 30 |
N total | 64 | 933 | 339 | 212 |
N pregnancies | 1 | 9 | 8 | 11 |
Pregnancy rate | 1.56% | 0.96% | 2.36% | 5.19% |
Confidence Interval | 0.04-8.40 | 0.44-1.82 | 1.02-4.60 | 2.62-9.09 |
At the recommended regimen, levonorgestrel is not expected to induce significant modification of blood clotting factors, and lipid and carbohydrate metabolism.
Paediatric population
A prospective observational study showed that out of 305 treatments with levonorgestrel emergency contraceptive tablets, seven women became pregnant resulting in an overall failure rate of 2.3%. The failure rate in women under 18 years (2.6% or 4/153) was comparable to the failure rate in women 18 years and over (2.0% or 3/152).