Actually, the carbon by-products from any kind of inhaled smoke cause your liver to make more of a specific kind of enzyme. This enzyme breaks down and inactivates drugs, especially hormones like birth control. Thus, the amount of active birth control in your blood decreases and is less effective. If you still smoke, you might think about asking your doc to switch to a higher-dose birth control.

Only cigarette smoke increases the risk for DVT, not marijuana smoke.

I'm a pharmacist, BTW.