We really do not know much about the pills pharmaceutical development. Whether half a day is half day light, 12 hours, 8 hours or what not we have no idea. We do not know much about the world of claymore as in how they measure time (no clocks) or whether their fictional world has the same time as we do. I am fine with everything not being explicitly described. The story is about relationships more than anything so as long as it stays focused on that other things being brushed over does not bother me. Remember when George Lucas decided to tell us the secret of the force in star wars was medichlorians? I'm just as happy not knowing in some cases
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As sleepy said pills might have been developed for stopping awakenings, but they might have been a complete failure. Or they are not a failure at all and the MiB's only use them for people in small doses because they want people to awaken. Or they were developed for the MiB's use originally. MiB's using them seems to make the most sense to me because they cannot sense youki but many have youki marks on their faces. Also their eyes are not silver. But once again, just a guess. Really we do not know much about the pills and they will probably have more information pop up about them in the future. For instance, the pills can be used for covert interrogations of claymores like Audrey as seen after Riful's encounter with the ghosts. It is also interesting to note that half a pill does not effect eye color, youki sensing or youki release as seen in Pieta. I think the pills will continue to be a major tool in the story.