In
hiv we give anti hiv drugs in suspicion of hiv contraction to the person because there is a window period.
Allergies are different scenarios.either you have them or you don't.
Do not mix your logic with medical science.Every disease has been studied and every treatment regime goes through intense scrutiny, experiment, trails before getting printed and taught in medical colleges.
Next Steps
If you have doubts kindly seek a medical professional who can clear you doubts.
Health Tips
Do not abuse medicine without professional supervision.It may cause more harm.than good.