How to obtain count of people who start off on one drug but then stop

[Merdock]

I have the following dataset where:

ID=participant ID

Visno=visit number

Visdate=visit date

Drug A, Drug B=indicators for whether participant was taking Drug A or Drug B

How can I get a count of how many participants who startoff on Drug A (Drug A =1 at visno=00) but then stop taking Drug A at some point after visno 00 without either switching to Drug B or re-starting Drug A at a later time point? For example, in this sample dataset I’m providing, this happens for IDs#001, 003 and 007 where they start on Drug A at baseline (visno 00) but then at some post-baseline time point, they stop Drug A without getting on Drug B or re-starting Drug A later on.

Thanks for any suggestions/help!

data have;
input ID$ visno$ Visdate:mmddyy. DrugA DrugB;
format Visdate mmddyy10.;
datalines;
001	00	2/24/2001	1	0
001	01	6/15/2001	0	0
002	00	10/23/2008	1	0
002	01	4/10/2009	1	0
002	02	11/25/2009	1	0
002	03	5/18/2010	1	0
003	00	9/22/2001	1	0
003	01	3/25/2002	0	0
003	02	7/30/2002	0	0
004	00	4/26/2001	1	0
004	02	3/19/2002	0	1
005	00	9/19/2002	0	1
005	01	2/20/2003	0	1
005	02	9/24/2003	0	1
005	03	2/6/2004	0	1
005	04	11/1/2004	1	0
005	05	1/14/2005	1	0
005	06	9/23/2005	0	0
006	00	7/28/2000	0	1
006	02	12/11/2000	0	1
006	03	6/26/2001	0	1
007	00	3/7/2002	1	0
007	01	8/14/2002	1	0
007	02	2/6/2003	1	0
007	03	8/27/2003	0	0
007	04	3/31/2004	0	0
007	05	9/15/2004	0	0
008	00	12/25/2002	1	0
008	01	5/29/2003	1	0
008	02	11/5/2003	0	0
008	03	4/23/2004	1	0
008	04	10/6/2004	0	0
009	00	4/24/2017	1	0
009	01	10/9/2017	0	0
009	02	5/21/2018	1	1
009	03	9/24/2018	0	1
010	00	2/27/2007	1	0
010	03	10/16/2007	1	0
010	05	3/25/2008	0	0
010	06	8/5/2008	1	1
010	07	6/2/2009	0	0
;
run;
proc print data=have;
run;

[Patrick · Accepted solution]

There might be more elegant ways but below should work.

data want;
  set have;
  by id visdate;

  retain _ind 0;
  if first.id then
    do;
      if drugA=1 then _ind=1;
      else _ind=-1;
    end;

  if _ind=1 and drugA=0 then _ind=0;
  if _ind=0 and (drugA=1 or drugB=1) then _ind=2; 

  if last.id then 
    do;
      if _ind=0 then output;
      _ind=0;
    end;
run;

proc print data=want;
run;

[Merdock]

Thanks Patrick! This works beautifully!