I put my pet's medication schedule on my computer calendar. I need to give my cat his medication once every three days so I set the reminder to come up every third day. It comes up in my e-mail and I don't erase it until I do it.
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I mark everything on my calender. When my pets' flea med is due, I mark it down. I do the same with meds such as heart worm med, I put a little heart on the calender.
I, or one of my family members, always give our dog her heart worm pill at the end of the month. By writing Jan., Feb., March, etc. on the back of each section of the blister pack, I can tell at a glance if she's been given her pill for that month.