Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency medical care now for a painful or rapidly spreading rash with blisters, peeling skin, or sores involving the mouth, eyes, or genitals.
This is especially important after starting a medicine within the previous several weeks.
Fever combined with skin pain, facial swelling, blistering, or a spreading dark-red rash also needs emergency assessment.
Do not drive yourself if you are severely unwell.
If TEN has already been diagnosed, hospital staff should be told immediately about trouble breathing, worsening confusion, reduced urination, new eye pain, or sudden changes in vision.
Contact your follow-up team sooner for increasing pain, pus, unpleasant wound odour, fever, worsening redness, or new vision symptoms.
After discharge, routine follow-up is used to monitor healing, review the suspected medicine, and check for lasting eye, skin, mouth, or genital problems.
Every sign above is quoted from the sofidex article on Toxic epidermal necrolysis. If you are not sure, err upward — it is always reasonable to be checked.