how to compare retention

[skk_2011]

how to compare the retention of 2 drugs, data is like:

Drug      week1    week2    week3 ...............................week20

drugA     100         82          55                                        26

drugB     810        580         460                                     312 

The numbers are the patient count who keep using the the drug starting week1.

question: retention statistically significant?

Thanks.