Tablets and capsules can be quite complex in how they are made. This is to make sure that the drug is released at the right speed into the body at the right location e.g. the mouth, stomach or intestines. The speed of release of the drug is controlled by the ingredients within the tablet which either hold it together or help it to break up easily. These coatings have a wide range of ways of working.
Before crushing a tablet you should always check with your pharmacist to see if it is appropriate. You can check your medicine on the database provided in this website to see if it has a special coating and also whether there may be other products such as liquids or patches available for you to use instead.
If your medicine is enteric coated and you crush it, you could be either increasing the chance of a stomach ulcer, reducing the amount of the drug in the body if it is broken down by acid in the stomach or stopping the drug from getting to where it needs to work. For this reason, you should never crush enteric coated tablets.
If your medicine is modified release, crushing it may cause the drug to all be released at once and therefore increase the chance of side effects and give you some time with no drug in the body at all. Again, modified release tablets should never be crushed.
If your medicine is film or sugar coated then it may be safe to crush it, however you should always ask the patient what the resultant mixture tastes like. If it is unpleasant then an alternative may be better.