Prometrium and Nicotine Interaction: What Smokers on Progesterone Need to Know

Does the Prometrium label say anything about nicotine?

The label's interaction information does not involve nicotine. The interaction subsection under Clinical Pharmacology reports one in-vitro finding: ketoconazole inhibited progesterone metabolism by human liver microsomes. The label says this suggests ketoconazole or other CYP3A4 inhibitors may increase progesterone bioavailability, and that the clinical relevance of the in-vitro findings is unknown [1]. The same subsection reports a study in 29 postmenopausal women where conjugated estrogens 0.625 mg with Prometrium 200 mg raised total estrone and total equilin and lowered circulating 17-beta estradiol [1].

The current label (Revision 02/2026) also has no boxed warning; the cardiovascular, breast cancer and probable dementia language was removed in the labeling update FDA approved on 2026-02-12 [7][1].

Is smoking a contraindication to Prometrium?

No. The label lists six contraindications: hypersensitivity to its ingredients (the capsules contain peanut oil, so peanut allergy is a contraindication); abnormal genital bleeding of unknown cause; known, suspected or past breast cancer; active or past deep vein thrombosis or pulmonary embolism; active or past arterial thromboembolic disease such as stroke or heart attack; and known liver dysfunction or disease [5]. Smoking is not among them [5].

A smoking warning does exist on combined hormonal contraceptives. The Sprintec label, for example, contraindicates combined oral contraceptives in women over 35 who smoke [8]. Prometrium is not a contraceptive, and smoking is not on its contraindication list [5].

Where the smoking and liver enzyme story comes from

Cigarette smoke induces CYP1A2, the hepatic enzyme that metabolizes caffeine, clozapine, olanzapine, theophylline and other drugs [3]. Polycyclic aromatic hydrocarbons in the smoke are considered responsible, acting through the aryl hydrocarbon receptor, and they induce CYP1A1 and CYP1A2 [2][9]. Nicotine is not the inducer: a review of smoking cessation and prescription drugs states that the effect on hepatic microsomal enzymes is not related to the nicotine component of tobacco, so nicotine replacement will not alter it [4].

One animal study complicates the picture slightly. A single nicotine dose to rats induced CYP1A1 in lung tissue, peaking at 14.7 times control activity 12 hours after dosing, but did not significantly change liver CYP1A2 protein or mRNA [10]. The human relevance of that finding is unknown [10].

Nicotine has its own metabolism: it is a CYP2A6 substrate, and estrogen induces CYP2A6, producing lower nicotine levels in women, especially those taking oral contraceptives [9].

Does smoking change progesterone levels?

The Prometrium label does not identify the enzyme that performs progesterone's main metabolism [1]. It describes hepatic metabolism largely to pregnanediols and pregnanolones, which are conjugated and excreted in bile and urine, and it mentions CYP3A4 only in the in-vitro ketoconazole sentence [1]. No study measuring progesterone concentrations in smokers taking Prometrium is available, so whether smoke-induced enzyme activity changes progesterone exposure has not been established.

What is documented is the effect of quitting on drugs known to be cleared by CYP1A2. In 12 heavy smokers who stopped abruptly, caffeine clearance fell 36.1 percent, from 2.47 to 1.53 mL/min/kg, with an apparent half-life of the CYP1A2 decline of 38.6 hours [11]. Caffeine half-life was 3.5 hours in 13 smokers versus 6.0 hours in 13 non-smokers [12]. Dose adjustment or monitoring is clearly indicated for warfarin, olanzapine, clozapine and theophylline, the CYP1A2 substrates with narrow therapeutic ratios [4]. Progesterone is not on that list.

Cardiovascular risk: the part that matters more

Nicotine may contribute to cardiovascular disease through the hemodynamic effects of sympathetic nerve stimulation and catecholamine release, but cigarette smoke contains many other cardiovascular toxins [13]. Cigarettes deliver nicotine faster and usually in higher doses than nicotine replacement, so the cardiovascular effects are more intense with smoking [13]. In trials of nicotine replacement in people with stable coronary disease, nicotine did not appear to increase cardiovascular risk, and at worst the risk was no greater than that of smoking [13].

The Prometrium label summarizes the Women's Health Initiative trial of 16,608 women aged 50 to 79 followed for an average of 5.6 years. Compared with placebo, the relative risks were 1.98 for pulmonary embolism, 1.87 for deep vein thrombosis, and 1.37 for stroke. That trial used conjugated estrogens plus medroxyprogesterone acetate, not micronized progesterone, and the label says the relevance of its findings to other products is not known [5][15].

On progesterone specifically, the ESTHER case-control study compared 271 postmenopausal women with a first idiopathic venous thromboembolism to 610 matched controls. It found no significant association with micronized progesterone (odds ratio 0.7, 95 percent CI 0.3 to 1.9), while oral estrogen was associated with a roughly fourfold increase (odds ratio 4.2, 95 percent CI 1.5 to 11.6) [14].

What this means in practice

Prometrium's labeled uses are narrow: preventing endometrial hyperplasia in postmenopausal women with a uterus who take conjugated estrogens tablets, at 200 mg at bedtime for 12 days per 28-day cycle, and treating secondary amenorrhea at 400 mg at bedtime for 10 days [5].

For a smoker, three things are true at once:

• The label does not list smoking as a contraindication or nicotine as an interaction [5][1].
• Smoke's enzyme induction is not caused by nicotine, so it ends when smoking ends, with or without nicotine replacement [4].
• Smoking has its own cardiovascular effects. The ESTHER finding on venous thromboembolism does not establish the overall cardiovascular safety of Prometrium [13][14].