Focus Addiction is a form of magic loss, which is really just a game mechanic to screw over munchkins who use truckloads of foci. You can have a number of foci active equal to twice your magic attribute. If you activate another focus over that, you risk magic loss. Divide the total amount of force for all active foci by two. That number is the target number for a Magic test. If that test fails, the character must check for magic loss.
To check for magic loss, roll 2d6, if the total of that roll is less than or equal to the character's Magic attribute, they lose a point of magic. When a character loses magic in this manner they must accept a geas or they can never recover that Magic point and can never initiate again, ever.
Magic loss is caused by several things: Deadly physical damage, disruption in astral combat (physical or stun damage), improper medical treatment (not applying the +2 target modifier for doctoring and first-aid), drug abuse (roll Magic vs. the drug's rating, usually, success means no Magic Loss check is made) losing a supremacy duel/being utterly defeated in a banishing contest, and of course, permanent essence loss, including but not limited to vampiric drain and cyberware. Drug addiction including smoking and drinking can affect magical ability too. If a character stays addicted to a substance they risk attribute loss and essence loss, which leads to magic loss.
Again, magic loss must be offset with a geas, there is no other way besides the path of the Burn-out. Unless you buy into a pact with a nasty Shadow spirit named Gaf who is said to be able to restore a burn-out.