Slappers were tiny skin patches that contained Venom.
Jackson Chappell learned the secret of Venom from an elderly Bane, so he could create the Venom Bane needed just to keep himself alive. Chappell used it to manufacture "slappers", patches filled with a moderate amount of Venom, and advertised them as highly addictive muscle enhancers in a black market scheme, typically selling them in packages. When used in moderation, they heighten speed, strength and aggression and are typically used by athletes as performance enhancers; when multiple Slappers are used at once, they create the same effect as Venom does when used by Bane. This has an advantage over the tubes Bane favored, as there is no way to sever the flow of Venom, which was a constant problem for Bane as Batman would tend to thwart him through destruction of the tubes. Due to Venom's addictive nature, Chappell could charge quite heavily, forcing people who couldn't afford it normally to have to steal to make the required credits to pay for them. Although Chappell made a fast buck from his "side business", he also attracted much undue attention with the crimes he was indirectly encouraging his customers to commit.
Despite their advantage to Bane's tubes in terms of supply, they do have severe, negative consequences: the slappers, after gradual usage, will have the user becoming more addicted, even for things that can be done normally. This is especially apparent with athletes for the Hill High Hawks hockey team members Mason Forrest, Dirk, and Jody, where they eventually became so addicted that they needed the slappers just to function. In addition, an overdose of slappers will result in the user experiencing extreme pain due to the swelling of their muscles, and the effects are painful enough that even something like being knocked unconscious has the high risk of leaving someone brain-dead. This is especially apparent with Chappell's ultimate fate when he was discovered to have been behind the production of slappers by Terry McGinnis, the new Batman.
After Forrest was arrested, this, combined with Chappell's own overdose of the drug in a fight with the new Batman, resulted in production of the slappers being halted.