If you buy diet pills online and are called by the Public Security Bureau to cooperate with the investigation, you'd better cooperate with the investigation. Because this may involve criminal offences or illegal acts, if you don't cooperate with the investigation, it may be regarded as refusing to accept the investigation and aggravating the crime.

If you are in doubt or not sure whether you should cooperate with the investigation, it is recommended that you consult a professional lawyer or relevant institutions for more accurate advice.