A drug delivery system comprising a first drug delivery device comprising: a first main portion comprising: - a first reservoir holding a first type of drug, - a first drug outlet, and - first drug expelling means for expelling drug from the first reservoir through the first drug outlet, characterized in that the first drug delivery device further comprises: a first cap releasably mountable to the first main portion to enclose the first drug outlet, the first cap comprising a first user-identifiable marking indicating the first type of drug, the system further comprising a second drug delivery device comprising: a second main portion comprising: - a second reservoir holding a second type of drug, - a second drug outlet, and - second drug expelling means for expelling drug from the second reservoir through the second drug outlet, a second cap releasably mountable to the second main portion to enclose the second drug outlet, the second cap comprising a second user-identifiable marking indicating the second type of drug, wherein the cap and main portion of each drug delivery device comprise a pair of corresponding key structures (421, 422, 423, 411, 412, 413) preventing the first cap from being mounted on the second main portion, and the second cap from being mounted on the first main portion, and wherein the drug delivery devices in the system are based on the same technology platform by having at least 50% interchangeable parts. a first main portion comprising: - a first reservoir holding a first type of drug, - a first drug outlet, and - first drug expelling means for expelling drug from the first reservoir through the first drug outlet, - a first reservoir holding a first type of drug, - a first drug outlet, and - first drug expelling means for expelling drug from the first reservoir through the first drug outlet, characterized in that the first drug delivery device further comprises: a first cap releasably mountable to the first main portion to enclose the first drug outlet, the first cap comprising a first user-identifiable marking indicating the first type of drug, a first cap releasably mountable to the first main portion to enclose the first drug outlet, the first cap comprising a first user-identifiable marking indicating the first type of drug, the system further comprising a second drug delivery device comprising: a second main portion comprising: - a second reservoir holding a second type of drug, - a second drug outlet, and - second drug expelling means for expelling drug from the second reservoir through the second drug outlet, a second cap releasably mountable to the second main portion to enclose the second drug outlet, the second cap comprising a second user-identifiable marking indicating the second type of drug, a second main portion comprising: - a second reservoir holding a second type of drug, - a second drug outlet, and - second drug expelling means for expelling drug from the second reservoir through the second drug outlet, - a second reservoir holding a second type of drug, - a second drug outlet, and - second drug expelling means for expelling drug from the second reservoir through the second drug outlet, a second cap releasably mountable to the second main portion to enclose the second drug outlet, the second cap comprising a second user-identifiable marking indicating the second type of drug, wherein the cap and main portion of each drug delivery device comprise a pair of corresponding key structures (421, 422, 423, 411, 412, 413) preventing the first cap from being mounted on the second main portion, and the second cap from being mounted on the first main portion, and wherein the drug delivery devices in the system are based on the same technology platform by having at least 50% interchangeable parts.