Because the Senate amended the original bill on the floor Monday night, the measure now goes back to the House for a concurrence vote. The House unanimously passed the bill last month.
The measure says that if less than an ounce of pot is found at a store with a pharmacy, the store must notify law enforcement, and the marijuana must be properly disposed of.
Friday night's amendment clarifies that the marijuana is destroyed or rendered unusable.
Supporters say the bill was prompted by recent incidents where marijuana was found at Walmart stores, after it was apparently dropped.
There was concern that having marijuana on the premises could affect the licensing of the pharmacies at such stores. Washington voters legalized possession of up to an ounce of marijuana last fall.