Oral Estradiol and Anesthesia: Perioperative Interaction Guide

Is there a drug interaction between oral estradiol and anesthetic agents?

Nothing in the oral estradiol tablet label (Teva, DailyMed version 16) describes an interaction with volatile anesthetics, propofol, opioids or neuromuscular blockers [1]. The label's drug interaction section is limited to a single enzyme family. Inducers of CYP3A4 (St. John's wort, phenobarbital, carbamazepine and rifampin) may reduce plasma concentrations of estrogens, possibly reducing therapeutic effect or changing the uterine bleeding pattern. Inhibitors such as erythromycin, clarithromycin, ketoconazole, itraconazole, ritonavir and grapefruit juice may raise estrogen concentrations and cause side effects [1]. The Climara patch label carries the same paragraph [4].

Why the label's perioperative concern is clots

Estrogens increase the risk of venous thromboembolism, and the oral tablet label states this in its warnings [1]. The figures in the label come from the Women's Health Initiative, which tested oral conjugated equine estrogens 0.625 mg, alone or with medroxyprogesterone 2.5 mg, not estradiol. In the estrogen-plus-progestin substudy the label reports a 2-fold greater rate of VTE, 34 versus 16 per 10,000 women-years [1]. The Climara label gives the estrogen-alone substudy figures, 30 versus 22 per 10,000 women-years, with only the DVT difference (23 versus 15) reaching statistical significance [4].

The two labels word the limits of the WHI data slightly differently. The oral tablet label says other doses, combinations and dosage forms were not studied in the WHI and that, without comparable data, the risks should be assumed to be similar [1]. The patch label says the relevance of the WHI findings to lower doses, other routes of administration or other estrogen-alone products is not known, and that these risks cannot be definitively excluded [4].

Both labels list active DVT or PE, or a history of either, as a contraindication. The oral tablet label also contraindicates active or recent arterial thromboembolic disease, such as stroke or heart attack [1]. The Climara patch label additionally lists a history of arterial thromboembolic disease and known protein C, protein S or antithrombin deficiency, or another known thrombophilic disorder [4].

What the label says about surgery

The oral estradiol tablet label's instruction reads: 'If feasible, estrogens should be discontinued at least 4 to 6 weeks before surgery of the type associated with an increased risk of thromboembolism, or during periods of prolonged immobilization' [1]. The Climara patch label carries a similar instruction [4].

Two parts of that wording matter. The window is 4 to 6 weeks, not a fixed 4, and it applies 'if feasible.' The trigger is not any operation but 'surgery of the type associated with an increased risk of thromboembolism' and periods of prolonged immobilization. The label does not specify which procedures fall into either category [1].

The label also says that if a cardiovascular event or VTE occurs or is suspected, estrogens should be discontinued immediately [1].

How large is the clot risk with oral estrogen?

The best-known observational study is ESTHER, a French multicenter case-control study of women aged 45 to 70 between 1999 and 2005, with 271 cases of first idiopathic VTE and 610 matched controls. Current users of oral estrogen had an adjusted odds ratio for VTE of 4.2 (95% CI 1.5 to 11.6) compared with nonusers. Current users of transdermal estrogen had an odds ratio of 0.9 (0.4 to 2.1) [2].

A 2008 systematic review and meta-analysis pooled eight observational studies and nine randomized trials. In the observational data, the odds ratio for a first VTE was 2.5 (1.9 to 3.4) for oral estrogen and 1.2 (0.9 to 1.7) for transdermal estrogen, and the oral risk was highest in the first year of use at 4.0 (2.9 to 5.7). In the randomized trials of oral estrogen the odds ratio was 2.1 (1.4 to 3.1) [3]. The ESTHER result and the route-specific odds ratios in the meta-analysis came from observational data, not randomized comparisons of oral and transdermal estrogen [2][3].

Oral tablets versus a transdermal patch

The Climara label notes that systemic availability of estradiol after transdermal use is about 20 times higher than after oral use, because there is no first-pass metabolism [4].

Observational data link oral estrogen, but not transdermal estrogen, to a higher VTE risk [2][3]. That is a consistent signal, not proof. The patch labels still list active or past DVT or PE as contraindications and warn about VTE [4].