Is pepcid AC effective after the expiration date?

Once a drug has past its expiry date the safety and efficacy of the medication can no longer be guaranteed by the manufacturer. Given that your condition is not life threatening take it if you wish - it might not work but it is unlikely to cause you any harm.
The best thing would be to throw it away and purchase a new box from your pharmacy.
For more information see: https://www.drugs.com/article/drug-expiration-dates.html