st: While/if for individual observations

["Williams, Barbara" <Barbara.Williams@vmmc.org>]

My data:
I have a list of 20,000+ patient visits to a clinic with the following 3 variables:

ID  VisitDate Antibiotics_YN
100 01feb2011 0
100 02feb2011 0
100 04feb2011 1
100 10may2011 1
100 10oct2011 1
101 01feb2011 0
101 12mar2011 0
101 13mar2011 0
101 14mar2011 0
101 18mar2011 1
...

My question:
I would like to create a new variable which labels Antibiotics WITHIN 28 days of their first visit.
AND If the patient has another visit more than 28 days after the first visit, the time is re-set (new series).
For ID=100 First series of visits beginning 01feb2011 Antibiotics on visitday = 4
                    Second series of visits beginning 10may2011 Antibiotics on visitday = 1
                   Third series of visits beginning 10oct2011 Antibiotics on visitday  =1
For ID=101 First series of visits beginning 01feb2011 No antibiotics
                   Second series of visits beginning 12mar2011 Antibiotics on visitday = 6


My syntax (doesn't run, I'm not sure how to do this):
local i = 1
local N = _N
while `i' <=`N'   {
   local firstdate = VisitDate[`i']
   while ID[`i'] == ID[`i' - 1] & VisitDate[`i'] <= firstdate + 28 {
       replace abxin28days[`i'] = 1 + abxin28days[`i']
                   local `i' = `i' + 1 }
   replace firstdate[`i'] = VisitDate[`i'] if _n==`i' 
   }

THANK YOU!

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