Have you ever eaten a supermarket egg? If so, in all likelihood, you've eaten an egg from a bird which was on medicated feed when the egg was laid. Its less prevalent than it was, but the close conditions and constant rotation of new birds thru a commercial egg laying facility strongly encourages the use of medicated feed to control coccidia - offset by the cost considerations of the medication itself. For its part, the FDA allows a "zero day" withdrawal period for chickens getting amprolium in accordance with package directions - so commercial egg producers can, and sometimes do, feed medicated to birds producing eggs for human consumption.
Most people choose to disconntinue use of medicated ffeds prior to start of lay, but its not required. the tiny amount of amprolium which makes it into the egg will not measurably block your thiamine absorption, unles s you eat so many eggs that the egg itslef become a danger - which occurs long before the amprolium reaches human-critical levels. Even then, a B-vitamin complex pill and you'd be fine.