its totally up to you , if its that chesty then you should go to the Dr incase its a chest infection and you need antibiotics , if its more of a cold type cough then most cough medicines will take you out of ketosis , so it depends whether you think they help your cough enbough to warrent breaking ketosis . Dont risk your health and if you need a break then take it , but weigh up the pros and cons first x hope you are better soon