Always check with a doctor or pharmacist before breaking up capsules and tabelst and mixing them with food or a drink. This is because some drugs come in capsules or have special coatings that are designed to make them dissolve slowly over a given time, to deliver a specific steady dose of drug through that time, or have coatings that resist dissolving in some parts of the gut because some drugs can cause unpleasant side effects or cause irritation in, say, the stomach. Breaking open the capsule, crushing the tablet and mixing it with food or dissolving it in a drink can circumcent this process.
A pharmacist or doctor can tell you if this is safe and appropriate to do. Alternatively, some drugs can be prescribed in a liquid form; less commonly, they may be available in tablets that dissolve in the mouth (called "oro-dispersible" form). Although the liquid and dispersible forms often taste just as nasty because they are so artificially sweet so as to disguise the nasty bitterness!
Diarrhea is a common side-effect of antibiotics. It usually happens because the antibiotics not only kill off the nasty bugs causing infection, they also kill off the nice "helper" bugs that naturally live inside our digestive systems. Everyone has these, in fact we could not live without them, they are essentialy to the digestive process - and they also help our immune systems by competing with harmful bugs that try to set up shop down there.
Sometimes, patients who have disrupted digestive systems (for example, long term treatment with powerful antibiotics) are given special capsules that contain "helper" bugs, in order to repopulate their guts with them.
Did you know the healthy human body has several kilos of bacteria in it, all the time?