Here is a quick trick you can use to rapidly assess the risk of drug to drug interactions:
Think of drugs that could possibly cause a serious adverse event that the patient might be taking. These typically are anticoagulants, diabetes medications that actively lower blood sugar levels, and cardiac medications that affect rhythm.
You should always do a quick drug interaction check when prescribing a patient a new medication, but do a more thorough interaction check when they are on any of the above medications.
Because the interaction could either adversely thin/thicken their blood, cause severe hypoglycemia, or put the patient into a dangerous arrhythmia. These are SERIOUS adverse events that could lead to death.
So, always do your due diligence when prescribing a patient a new medication who happens to be on any of these types of medications. If they are not on these classes of medications, then typically the drug to drug interaction is low.
As always, use your own clinical judgement and understand the pharmacology of the medications you are prescribing.