to the editor: The article “Update on Oral Contraceptive Pills”¹ was timely and informative. However, one critical piece of information was overlooked.

When prescribing oral contraceptive pills (OCPs), particularly when they are to be used as “emergency contraception,” it is important for physicians to remember that one of the potential mechanisms of action is prevention of implantation.² Given that the United States has a diverse population, it is probable that a substantial number of women seeking birth control or “emergency contraception” subscribe to the belief that the fetus is a human being from the time of conception, and that it is morally wrong to interfere with the life of that human being for any reason, with the possible exception of a significant danger to the life of the mother. Therefore, it is incumbent on physicians who prescribe OCPs to review the possible mechanisms of action with all patients, so that those who would choose not to take the chance of allowing conception but preventing implantation, can make a truly informed decision. This is particularly important when prescribing OCPs for “emergency contraception,” where the probability of failing to prevent ovulation and successfully preventing implantation is much higher than with the usual use.

It should be noted that “emergency contraception” is an unsatisfactory term because it may give the impression to the lay person that conception is reliably being prevented. Conception may be prevented when OCPs or Preven (levonorgestrel and ethinyl estradiol) are used, but, as noted earlier, prevention of implantation is also a possible mechanism of action. Depending on the timing of administration of the medication, it may even be a more likely mechanism. Prevention of implantation is nearly certain when an intrauterine device is used, making “emergency contraception” an even more inappropriate term.

to the editor: The article “Update on Oral Contraceptive Pills”¹ provides a fairly thorough overview of many of the issues surrounding the prescribing of this form of fertility control. It did not, however, include certain information on oral contraceptive pills (OCPs) that is essential for at least some patients, to make an informed choice on the use of OCPs that is consistent with their values. That information concerns the mechanisms of action of these agents.

While the predominant mode of action of combination hormonal contraceptives is that of ovulation suppression, this effect is not total, with some degree of breakthrough ovulation occurring in up to 10 percent of cycles.^2,3 Other mechanisms of action (changes in the cervical mucus and endometrium) are recognized⁴ and mentioned in the prescribing information.⁵ The endometrial changes make implantation less likely,⁴ which is a mechanism that occurs after fertilization. It is unknown how often a postfertilization effect occurs, but some estimate can be made by examining the ratio of ectopic to intrauterine pregnancies. This ratio is elevated in women who take OCPs versus those who do not.⁶ Even if the likelihood of this occurring in any one cycle is low, the likelihood that a patient will eventually encounter this mechanism over years of OCP use would be high.

For a patient who is of the conviction that human life begins at fertilization, these effects are serious concerns. I am aware of patients with such convictions who were placed on OCPs without any mention of this by the prescriber, and who were later quite upset when informed of the potential for a postfertilization mode of action.

I believe it is imperative for all physicians who prescribe OCPs to provide information about the potential for a postfertilization effect. Patients can then make a more fully informed choice about fertility control options. Certainly, family physicians, who aim to treat the whole person, should seek to be aware of situations in which prescribed drugs may not be consonant with the value systems of our patients.

editor's note: These letters were sent to the authors of “Update on Oral Contraceptive Pills,” who declined to reply.