These two words are both adjectives, and they are both related to the verb 'compel', which means to force someone to do something. However, the two adjectives do describe different things, and do have slightly different meanings.If something is compulsory, it is something that must be done because of the law, or because someone orders you to do it. For example:
In contrast, we use compulsive to describe someone's behaviour. It means that it is very hard for that person to stop doing something. For example, someone may be a 'compulsive' eater, which means that he or she cannot control their eating habits; they eat all the time, even when they are not hungry. In general, we use compulsive with behaviour that we think is negative, or which is the result of psychological problems. So, there are 'compulsive' eaters, 'compulsive' gamblers and 'compulsive' liars.There is one other use of the word compulsive, which is slightly different. We can describe a book as 'compulsive' reading, or a film as 'compulsive' viewing. This means that it is so good that you cannot stop reading or watching it.Interestingly, we can also say that a book is a 'compulsory' read, and this means that the book is a very important one and everyone should read it. Students at university may get a reading list for their course, and some of the books will be 'compulsory' reading, while others may be optional. Obviously, the books in the compulsory category are the ones that the lecturers think the students must read.
Gareth Rees has a BA (hons) in History and Philosophy of Science, CTEFLA, and DELTA. He has taught EFL, EAP and Business English in China, Spain and England, and he is the co-author of the Language Leader Elementary and Pre-Intermediate English language course books (Pearson Longman). He currently teaches English in the Language Centre at the University of the Arts, London.