Clomifene (INN) or clomiphene (USAN and former BAN) or Clomid or Clomifert is a selective estrogen receptor modulator (SERM) that increases production of gonadotropins by inhibiting negative feedback on the hypothalamus. It is used mainly for ovarian stimulation in female infertility due to anovulation
Letrozole (INN, trade name Femara) is an oral non-steroidal aromatase inhibitor.Estrogens are produced by the conversion of androgens through the activity of the aromatase enzyme. Estrogens then bind to an estrogen receptor, which causes cells to divide.
Letrozole prevents the aromatase from producing estrogens by competitive, reversible binding to the heme of its cytochrome P450 unit.Letrozole has been used for ovarian stimulation by fertility doctors since 2001—having less side-effects than clomifene (Clomid) and less chance of multiple gestation. A Canadian study presented at the American Society of Reproductive Medicine 2005 Conference suggests that Letrozole may increase the risk of birth defect. A more detailed ovulation induction follow-up study found that letrozole, compared with a control group of clomiphene, had significantly lower congenital malformations and chromosomal abnormalities at an overall rate of 2.4% (1.2% major malformations) compared with clomiphene 4.8% (3.0% major malformations).
A recent meta analysis comparing the two treatments for ovulation induction, Clomid vs Femara , published on RBM online, suggests that the two treatments are equivalent outcome-wise. Side effects seem to be less for the Femara group.
In my experience the response seem to be very individualized : some patients respond better to Clomid , others to Femara.
AbstractThe aim of this study was to systematically compare the clinical efficacy and safety of letrozole with clomiphene citrate for ovulation induction in women with polycystic ovary syndrome (PCOS). The Cochrane Central Register of Controlled Trials, PubMed, EMbase, CBMdisc and CNKI were searched for eligible randomized controlled trials (RCT) comparing letrozole with clomiphene citrate in PCOS patients. Two reviewers independently extracted information and evaluated methodological quality according to the Cochrane Handbook 5.0. Meta-analysis was performed with the fixed-effects model or random-effects model according to the heterogeneity. Six eligible RCT involving 841 patients were included. Letrozole was associated with a number of lower mature follicles per cycle (standardized mean difference, SMD, –1.41; 95% CI –1.54 to –1.28; P < 0.00001) compared with clomiphene citrate. There were no significant differences in pregnancy rate (relative risk, RR, 0.97; 95% CI 0.79 to 1.18), abortion rate (RR 1.38; 95% CI 0.48 to –3.96) and multiple pregnancy rate (RR 0.34; 95% CI 0.07 to –1.72) between the two groups. The evidence from ovulation rates was not enough to support either letrozole or clomiphene citrate. In conclusion, letrozole is as effective as clomiphene citrate for ovulation induction in patients with PCOS