Reason for caution regarding Plan B

In our continuing debate over whether the Vatican overstated the case against Plan B, "Reader 1" responds to "Reader 2":

First, the person who quotes Sulmasy's article overstates the degree of our certainty about how Plan B works - that is, he or she claims that it is only a "possibility (at most, and rarely)" that Plan B prevents implantation.  I'd be very happy to be mistaken, but I don't think the data supports this conclusion. [The manufacturer notes that Plan B may inhibit implantation.]

Second, I think that we would have to know more about the intention of women taking Plan B to conclude that the embryo's death (in the case of blocked implantation) is an indirect, foreseeable but unintended result.  I would imagine that most people who take the pill are simply intending not to become pregnant and don't think much about how the drug works.  The political and commercial campaign for Plan B simply says "it will not disrupt a pregnancy" -- failing to note the morally significant point that it nevertheless might cause the death of a living human embryo that has yet to implant.  If a woman considering using Plan B consults the FDA website about its function, she will find the following explanation:

3. How does Plan B work?

Plan B works like other birth control pills to prevent pregnancy. Plan B acts primarily by stopping the release of an egg from the ovary (ovulation). It may prevent the union of sperm and egg (fertilization). If fertilization does occur, Plan B may prevent a fertilized egg from attaching to the womb (implantation). If a fertilized egg is implanted prior to taking Plan B, Plan B will not work.  

Again, I would reiterate that if one regards intentionally preventing an embryo's implantation in the womb (and thus causing its death) as morally tantamount to intentionally preventing a living person from receiving needed life support, then it strikes me as sensible to think carefully before acting in this context.  I find it surprising that the person quoting Sulmasy is arguing against proceeding cautiously in the face of a known grave risk.  Surely caution and reflection are crucial features of medical practice.  And since Plan B was approved for over-the-counter use in 2006 (for people over the age of 17), it seems likely that most people self-administer the drug without the advice of a physician.  So comparing this to the "clinical context" seems inapposite.

Also, another reader writes that "Reader 1" did "not include the most recent research on Plan B,
which has not found evidence for post-fertilization effects. Researchers believe that the effectiveness of Plan B can be explained entirely by pre-fertilization effects, and that Plan B is less effective [in] preventing (clinical) pregnancy than was previously believed. Rich Poupard from the Life Training Institute summarized the evidence in a series of posts in 2007 and 2008."