A system comprising first and second drug delivery devices (1) based on the same technology platform, each comprising a main portion (3) with a reservoir holding a type of drug, a drug outlet, and drug expelling means for expelling drug from the reservoir through the drug outlet, as well as a cap (100) releasably mountable to the main portion to enclose the drug outlet, each cap comprising a unique user-identifiable marking indicating the corresponding type of drug. The cap and main portion of each drug delivery device comprise a pair of corresponding key structures preventing the first cap from being mounted on the second main portion, and the second cap from being mounted on the first main portion.