Which contraceptive pills are suitable for patients taking hepatic enzyme-inducing medicines?
UK Medicines Information summarises the evidence for this frequently asked question: What is a suitable combined oral contraceptive pill for a patient who is taking medicines that induce hepatic enzymes?
Combined oral contraceptive pills (OCPs) have a higher failure rate in women who are also taking hepatic enzyme-inducing medicines, for example antiepileptic drugs (AEDs).
AEDs such as carbamazepine, oxcarbazepine, phenytoin and topiramate induce hepatic cytochrome P450 3A4 enzymes, increasing the rate of combined OCP hormone metabolism and reducing circulating levels of these hormones by as much as 50%.
In addition, carbamazepine and phenytoin increase the plasma levels of sex hormone binding globulin, which may decrease OCP efficacy further.
A daily dose of oestrogen (ethinylestradiol) of 50–60µg is required for efficacy in these patients. Most UK-licensed combined OCPs do not contain this amount of oestrogen.
Although unlicensed, a combination of monophasic, lower-dose pills can be used to obtain the required 50–60µg of oestrogen. The two products chosen also need to contain the same progestogen (eg, using Loestrin 20 and Loestrin 30).
The usual seven-day pill-free interval each month also weakens the contraceptive effect. Therefore, these patients should take three or four strips of monophasic tablets without a break (“tricycling”), followed by a short pill-free interval of four days.
Tricycling and dose-doubling of combined OCPs as described above are unlicensed uses. It is important that the patient is informed of this. The patient should also be made aware that there is uncertainty about the effectiveness of this regimen.
The preferred method of contraception for these patients is an intrauterine device or injectable medroxyprogesterone acetate.
This FAQ is taken from a “Medicines Q&A” produced by UK Medicines Information. The full document, including references, is available online
Date prepared: 16 September 2010
Date expires: 16 September 2015
Citation: Clinical Pharmacist URI: 11040100
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