Can i cut diphenhydramine in half?

Depends: Some Diphenhydramine comes in capsules and some in solid pills. You can cut the pills but the capsules will just squish, break open and spill the contents. If capsules are all you have, you can dissolve the contents in water and drink half or something we do with patients at times is to mix it with jello and have them eat the proper amount.

Tabs, yes: It is not practical to cut a capsule, but a scored tablet can be easily cut with a pill cutter. There are many formulations of Diphenhydramine in case a lower than usual dose is required.