Bacitracin interferes with the way certain bacteria build their cell walls. Without an intact wall the bacterium cannot hold itself together and dies. It works mainly against gram-positive bacteria, a group that includes staph and strep.
That sounds like it should make it a good treatment for skin infections, and in the laboratory it looks reasonable. In practice two things get in the way. Resistance among Staphylococcus aureus is now widespread, so the bacteria most likely to be causing an infected patch of skin are frequently the ones bacitracin cannot touch. And in head-to-head trials for impetigo it has performed clearly worse than mupirocin.
Most of what people experience as bacitracin working is not the antibiotic at all. It is the ointment base. A wound kept moist and covered heals faster, with less scabbing and a better scar, than one left to dry out — and plain petroleum jelly does that just as well.
Stop it if a rash develops. Redness, itching, small blisters or weeping appearing three to seven days into using it is the classic pattern for allergic contact dermatitis, and it is very frequently mistaken for the infection getting worse — which leads people to apply more of the thing causing it.
Avoid triple-antibiotic products containing neomycin if you have eczema or a history of reacting to topical products. Broken skin absorbs more and sensitises more readily.
Do not use it on large areas, deep wounds or extensive burns. Absorption through broken skin is what makes bacitracin allergy — including rare cases of severe whole-body allergic reactions — possible.
Keep it out of the eyes and off the ear canal if the eardrum may be perforated.
Do not use it as a substitute for a proper antibiotic on an infection that is spreading, with redness moving outward, warmth, swelling or fever.
Food, alcohol, sun and oral medicines are not an issue.
None. There are no tests and no follow-up for an over-the-counter wound ointment.
The one thing worth doing is remembering. If you have had a rash from bacitracin, neomycin or a triple-antibiotic ointment, write it down and tell any doctor before a procedure — these ointments are routinely applied after surgery, and a reaction on a fresh wound is a genuinely miserable complication. If the reaction was clear-cut, patch testing can confirm which ingredient it was.
A clean minor wound kept moist and covered closes over roughly three to seven days, whether you use bacitracin or petroleum jelly.
On an actual infection, expect little. If a crusted, weeping or spreading patch has not improved in two to three days on bacitracin, it will not improve on day five either — that is the point to get a prescription rather than continue.
The timeline that catches people out runs the other way. Allergic contact dermatitis to bacitracin usually appears three to seven days in, sometimes later, and it looks like the wound getting angrier. If things were settling and then turned red and itchy around day four, suspect the ointment before you suspect the bacteria.
Nothing happens when you stop. There is no rebound, no taper, no course to finish.
If you stopped because of a rash, the rash takes one to three weeks to settle after the ointment is gone, and it may need a short course of a topical steroid to calm down. It does not settle while you are still applying the ointment, which is the trap.
And once you are sensitised to bacitracin or neomycin, you stay sensitised. It is a lifelong allergy, and it will react to the same ingredients in a different brand — including in ear drops, eye drops and post-surgical ointments. Worth writing on your allergy list.