If they are prescription, no limit, but they must have their labels on the bottles clearly stating what they are, who they're prescribed for, the name of the dispensing chemist, and the person prescribed the tablets must be travelling with them. Then there should be no issues.
Problems arise when you take what is deemed excessive, if you travel for 3 months and take 4 months supply this is a reasonable contingency to cover any emergencies such as a vehicle breakdown or possible accident resulting in a hospital stay, but take one years worth and this would be deemed excessive. Under such circumstances you would be investigated and if you can't provide a reasonable explanation for so many drugs you would most likely be classed as a smuggler, or if the drugs can be adapted or modified, a drugs dealer.
Friend at Customs suggests the following, keep all medication together, including prescription and non prescription, ensure all the labels are clearly visible with all the relevant information, and possibly in one clear plastic box so they cannot be deemed to be hidden. Take a list printed out on a seperate sheet of ALL medication, the quantity, types where applicable, and sizes of tablets; but keep this seperate from the drugs so you can refer to it if they are lost or stolen.
Keep all tablets, particularly prescription in the box or packaging they come in, and keep them in your hand luggage rather than a suitcase, if you are delayed you have easy access to them, and you are not hiding them.
Smuggling of non prescription drugs and tablets apparently, is big business, and many countries have excesively high prices for over the counter tablets we can buy cheaply, one example is Scandanavian countries where aspirin is massively expensive and many regular travellers take cases full of them, this is illegal and can land you in jail.