I don't know - but sometimes there is little choice because a medication dose not come in a low enough dose...
A good example is a fairly recent one with me - I was asked to give Paxil a try, but due to how sensitive I seem to be to side-effects so that general type of drug, he wanted to start with 5mg - but the smallest dose made is 10mg pills - but there is a score in the center...Even at half a pill - I had to stop due to bad effects....