Will Keppra deactivate or cause Plan B to not work?

There are no listed problems/interactions between Keppra & Plan B (Levonorgestrel) two drugs when you put their names into this website's checker.

Keppra does not have any effect on hormone breakdown, so I'm unaware of the reason for your neurologist telling you that you can't take birth control pills. You'd have to ask.

One group of anti-seizure medications are known as liver enzyme inducing. They increase the rate the liver breaks down contraceptive hormones. Therefore can effect your hormonal birth control. Keppra is not one of those.

There is another group of anti-seizure medicines which can increase your hormone levels, & therefore your doctor has to monitor you on one of those, in case you end up with too much of a hormone in your body, when including hormonal birth control. Keppra is not one of those.

Then there is the group that has no effect on hormone breakdown, & Keppra is one of those.

Perhaps someone who knows more than I do, will come along to answer for you.