If you suspect that your co-worker is under drug influence, it is important that you report it your manager or supervisor. Your supervisor or manager will be in better position to take appropriate action. For the manager, the first task will be to observe the employee's behaviour.
As a manager, your first aim should be to counsel your employee, where you inform him or her about the inappropriate conduct and the need to stop using drugs or alcohol during office hours. This is important as he may be a valued employee of the company. Advise him or her on taking proper professional help or joining the company’s rehabilitation program, if it is in place. Moreover, convey the risk associated of being under the influence of alcohol or drugs – firing, legal action etc.
However, if the problem is not solved, then take necessary measures. These will include creating a detailed report and sending it to the human resource department which will then look into the case. A company may conduct a random drug test at this point in order to save itself from incurring any liabilities if the employee was accused wrongfully. However, if such policy is not in place, then make certain that your employee uses drugs before pursuing the case any further.
In cases, where you cannot report the incident to your supervisors, due to the fear of being exposed to unwanted hype, attention or even retaliation, you can contact your company by pretending to be an anonymous caller. This way you are further helping your co-worker as the company cannot fire him or her until they provide the option of treatment or recovery.