It is not quiet a automatic as anything beyond first aid. There are other factors to consider whether it is work related. If it is due to a non-work related illness that manifests at work, it is not work related. However, OSHA has been pretty consistent in considering substance abuse in the work place something that the employer is supposed to control.
This is from OSHA's recordkeeping manual:
Question 7-9 If an employee who sustains a work-related injury requiring days away from work is terminated for drug use based on the results of a post-accident drug test, how is the case recorded? May the employer stop the day count upon termination of the employee for drug use under section 1904.7(b)(3) (vii)?
Under section 1904.7(b)(3)(vii), the employer may stop counting days away from work if an employee who is away from work because of an injury or illness leaves the company for some reason unrelated to the injury or illness, such as retirement or a plant closing. However, when the employer conducts a drug test based on the occurrence of an accident resulting in an injury at work and subsequently terminates the injured employee, the termination is related to the injury. Therefore, the employer must estimate the number of days that the employee would have been away from work due to the injury and enter that number on the 300 Log.
It does not specifically address the work related question, but obviously assumes that it is.