Last month, show bosses announced that Ryan would be returning to the cobbles with new recruit Sol Heras playing the part. The role was recast after original actor Ben Thompson opted not to return to the ITV1 soap.
A comeback for Ryan had previously been rumoured for some time, with reports suggesting that he would feature in a drugs storyline with Sophie Webster.
Marsh has now told All About Soap that Ryan will indeed be a drug user when he returns to Weatherfield.
She explained: "Ryan's been off doing his own thing, and has got heavily involved in drugs. Michelle expected him to come back as the same kid he was before he went away, but that's not going to happen."
Asked how difficult this experience will be for Michelle, the actress replied: "Very hard. Michelle's got her work cut out trying to get Ryan back on the straight and narrow, but we'll see her get support from outside influences."
Ryan's return scenes will air in July.