Patients should be asked if they have had previous self-poisonings. If they have, you should attempt to find out what drugs and doses the patient poisoned themselves with on those occasions as well as their current regular treatment, etc. This can be helpful if a history is unavailable or unreliable on this occasion.
A history of previous self-harm can also be an indication of suicidal intent. For example, patients who present consistently taking 10 grams of paracetamol generally do so with the knowledge that they came to no harm last time. On the other hand, patients who present initially with 10 grams of paracetamol, then 15 grams, then 20 grams, then 25 grams, etc., are endeavouring to establish the correct dose for the desired response, which may be death or hospitalisation or other outcomes. This is an n=1 dose-response toxicology study.