I would advise keeping  canula... less painful, more ideal and proper way of giving injection... use waterproof 3M cavafix for fixing the canula , ask the person who gives injection  to clean hand with medical grade hand sanitiser before touching  canula ,, clean canula with hand sanitiser These all will help preserve the canula longer Also the antibiotics  should be diluted in 100  ml ns wherever  required to avoid  pain and inflammation Etc