True but people in general fear needles. HIV patients are used to needles (blood tests etc.), healthy women are not. So the last group is inclined to chose pills over injection. The injection itself is therefore less of a problem for HIV patients. But as misiu states it's painfull and there is a longer term after effect. Well, in that case...

I truly believe HIV patients, having a chronic disease, are better educated and will mostly go for the med with the least side effects and not (necessarily) for an injection with the least frequency.