How do I tell a normal reaction from something that needs a doctor?

Asking generally rather than about myself, because I think this is the thread this category most needs and I could not find it.

Small red bump after injecting, mildly sore. At what point does that stop being normal?

The single most useful discriminator is not what it looks like now. It is which direction it is going over hours.

A normal reaction peaks early and then settles. Sore at the time, maybe a small red area, maybe a bruise, noticeably better the next day, gone in two or three.

The things that need attention get worse over hours to days:

• Redness spreading outward, especially with a defined advancing edge. Marking the border with a pen and looking again in two hours is the oldest trick there is and it works — you are converting "is it spreading?" into a yes or no.
• Increasing rather than decreasing pain, particularly pain out of proportion to how it looks.
• Warmth spreading beyond the immediate site.
• A firm, tender, deepening lump — an abscess collecting.
• Fever, chills, or feeling systemically unwell. This one changes the category entirely. Local problem plus feeling ill is not a local problem any more.

The injection site guide has the full version and it is the guide I would read even if you read no others.

Separately, and this is the part I want in its own post so it does not get buried in a list:

Some things do not wait. Difficulty breathing, swelling of the face, lips or tongue, a rash spreading over the body, feeling faint — that is an anaphylaxis picture and it is an emergency call, not a next-morning GP appointment.

Same for spreading redness with a fever, or severe pain out of proportion to appearance. Serious soft tissue infections progress in hours, and the difference between treating one early and late is very large.

Nobody has ever regretted going in and being told it was fine.

Two practical additions from long experience.

Tell them what it was. Whoever you see needs to know what was injected, when, and where, and this is the bit people fudge because they are embarrassed. Do not. They are not there to judge you and they cannot treat you properly without it. "An unlicensed peptide I bought online" is a complete and useful sentence and they will have heard it before.

Photograph it. Same light, same angle, with the date. Progression over twelve hours is the diagnostic information and human memory for "was it this red yesterday" is terrible. It also gives a clinician something objective.

And the one that is neither infection nor allergy and gets missed: lipohypertrophy. Repeated injection into the same spot causes fatty tissue to thicken over weeks to months.

It is not dangerous in itself. It matters because absorption from a lipohypertrophic site is erratic, which means whatever you thought you were administering, you were not. Rotation exists for this reason rather than for comfort.

You can usually feel it before you can see it — a rubbery thickening under the skin.

The pen mark trick is the concrete thing I was after. Turns a judgement call into an observation.

This is the thread to link when this comes up, and it will. Nobody diagnosed anybody, which is right — the useful move is giving somebody the discriminator and the threshold for going in.