Let me explain why one should not divide the heartworm pill. Most heartworms pills, in order to be more user friendly, come in a flavored treat. The way flavored heartworm pills are made make this a big no no. First the flavor part of the pill is made, which is virtually the whole pill. Next the flavor pill travels down an assembly line and the actual heartworm prevention is injected into the pill. In doing this, the medication is not evenly distributed through the whole pill. Therefore, when you half the pill one portion may have all the medication and the other may have none. If you are giving half a pill each month, you have then left the dog unprotected for one month.
So please, I beg of you. Be cheap another way.