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 care for chest pain, fainting, severe breathing difficulty, or signs of a serious allergic reaction while using a treatment.
Get urgent mental health support if you develop suicidal thoughts or feel unable to keep yourself safe.
Arrange a routine appointment if you are unsure why your hair is thinning, want to discuss treatment, or are distressed by the change.
Assessment is also worthwhile before paying for commercial hair-loss services.
The article also says
Seek a timely medical assessment if the loss is:
Sudden, very rapid, or occurring in round patches
Affecting eyebrows, eyelashes, or body hair
Accompanied by pain, burning, redness, heavy scaling, sores, or pus
Associated with fatigue, unexplained weight change, or other new symptoms
These features are not typical of male pattern baldness and may indicate another condition.
Early assessment is particularly important if the scalp appears scarred or shiny because some inflammatory conditions can permanently damage follicles.
Hair loss itself is not an emergency.
Every sign above is quoted from the sofidex article on Male pattern baldness. If you are not sure, err upward — it is always reasonable to be checked.