In general, it is okay to break or open a vitamin or other supplement, either by splitting or crushing a tablet, or twisting open a capsule, unless directions say not to or if it is an extended-release, timed-release, or enteric coated pill.
The best way to break or crush pills
Capsules are usually two-part shells, with their open ends facing one another. Twisting the capsule will separate the two. If you want to reduce the dose of a capsule without wasting the remainder, you can purchase inexpensive empty capsules, twist them open, and pour a portion of the contents from the original capsule into one of the empty capsules and then reclose each capsule.
If you want to break a tablet, you can purchase a relatively inexpensive pill cutter to more precisely cut a tablet into parts.
You can also use a pill crusher or mortar and pestle to crush a tablet into a powder that can be taken with food or in a drink. If directions say to take the tablet on an empty stomach, don't mix the crushed tablet with anything other than water. Either way, be sure to consume all of it to get the complete dose, and do this right away to minimize degradation of ingredients due to exposure to air and compounds in food or drink.
While you won't want to break a softgel in half, since it is a one-piece shell that typically contains a liquid ingredient, you can puncture the capsule, squeeze out the contents, and swallow the liquid directly or add to food or a drink.
Be aware that depending on the contents of a pill, it may have an unpleasant taste, which was previously masked.
Pills that should not be broken or crushed
If a pill is labeled as enteric-coated (to protect contents from stomach acid or avoid an unpleasant aftertaste, such as some probiotic and SAMe supplements), timed-release (to slow the absorption of a key ingredient, such as multivitamins, B vitamins, red yeast rice, and melatonin), or indicates some other type of special coating, you should not break it, as you will lose the benefit of the coating. As noted earlier, some coatings also mask the initial unpleasant taste of an ingredient.