Simulate effect of a time-varying categorical covariate

[melmelie]

Dear Ron,
I am trying to do a simple simulation for a cross-over trial where in period 1 multiple doses of Drug X alone (or under fasting condition) are administered. In period 2, multiple doses of Drug X are administered in combination with a another conmed (or with food). How can I code the off/on switch of the drug-drug or drug-food interactions (basically being silent in Period 1 and then active in Period 2)?
Thanks for your reply in advance...
Mohamed