There was a significant drug interaction with Efudex Cream that I was applying topically to a pre-cancerous 2-cm-diameter lesion on my left shin. On the day of my first Pfizer shot, the Efudex treated lesion was progressing normally from the desquamation (scabbing and shedding of epithelial scales) to the erosion stage (starting to form a shallow 2.5-cm-diameter ulcer). Starting about Saturday 30 January (three days after the first Pfizer shot) and slowly increasing for the next four days, the Efudex-treated ulcer became deeper, the peripheral 12-cm-diameter skin became redder, and the left foot and ankle became increasingly swollen.  My RN-wife and I (a former medical-research physicist) began research andChronic health conditions: nasal polyps, precancerous bleeding actinic keratoses, Cramp-Fasciculation Syndrome (leg & foot cramps), early macular degeneration bilateral, hearing loss bilateral. Chronic health conditions: nasal polyps, precancerous bleeding actinic keratoses, Cramp-Fasciculation Syndrome (leg & foot cramps), early macular degeneration bilateral, hearing loss bilateral.DVERSE EFFECTS, TREATMENT, AND OUTCOME:  discovered that Efudex should not be given concurrently with any of a long list of immunizations.  This is an Efudex problem that should gain another immunization onto that list. Being within a week of finishing the Efudex regimen, we monitored the Efudex-treatment area and the peripheral area carefully. On Thursday 4 Feb (eight days after the first Pfizer shot), the ulcer began unusual drainage, the 12-cm-diameter peripheral area became inflamed, and swelling of the left ankle and foot increased to almost skin-tight - I can upload photos if you need.  Time for immediate action.  My excellent Dermatologist stopped Efudex and started me on Bactrim by mouth for 10 days, and Gentamicin topical antibiotic on the ulcerated central area and Triamcinolone topical corticosteroid on the peripheral and swollen areas for three weeks.  Good choices.  Improvement was immediate and continued improving for the next three weeks.  By the time of the second Pfizer shot on 24 February, the central area was healing well, in the late-reepithelialization stage (regrowth of epithelial tissue that now formed a completely-covering scar tissue with no oozing), and with all inflammation and swelling completely gone.  After the second Pfizer shot (taken 20 days after stopping Efudex), there were zero adverse effects of any kind. NOTE: There were absolutely zero other adverse effects of any kind throughout the entire series: no chills, fever, headache, nausea, vomiting, diarrhea, change in pulse or blood pressure (120/70 range), loss of taste or smell, sore throat, cough, lung congestion, fatigue, feeling of being unwell, shortness of breath, injection-site swelling or redness, hives, swelling of face, decreased kidney function, or other adverse cardiovascular, neurological or pulmonary effects. There was no additional joint pain above my normal osteoarthritis.  After both shots, I felt a slight normal-range injection-site soreness for the next few hours, but no site swelling, rash, redness, or other problem.  We recommend that this immunization be added to the Efudex  proscribed or warned' list.