If you have data for those records make a 'master table' that has the values for the drug/year and then merge the tables on drug/year.
You can probably use PROC STANDARD for this to replace missing values, as long as you're okay with using the mean value of the drugs per year. If not you'll need another method.
http://support.sas.com/documentation/cdl/en/proc/61895/HTML/default/viewer.htm#a002473725.htm
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