Many people ask why the NovoPen series, including the NovoPen 4, NovoPen 5, NovoPen 6, and Echo Plus, is different from most other insulin pens. The reason comes down to cartridge design.

Novo Nordisk uses its own Penfill cartridge format rather than the more common standard 3 mL cartridge. Penfill cartridges are slightly thinner, slightly longer, and include a plastic male thread attached to the neck of the cartridge. In most other insulin pen systems, the threads are built into the cartridge holder, while the cartridge itself remains unthreaded.

Because of these differences, standard 3 mL cartridges do not fit NovoPen series pens without modification. Two members of the community, PeppyThePrawn and Max Headroom, independently developed solutions to work around this compatibility issue.

PeppyThePrawn’s Original Solution

PeppyThePrawn was first to bring a working solution to market. His approach modified the original cartridge holder by grinding the inside to allow a standard 3 mL cartridge to fit. The design also used a 3D-printed threaded adapter for the cartridge neck, along with a spacer to account for the cartridge length difference.

Although this solution required manual modification by either the vendor or the customer, it was the first successful method for using standard 3 mL cartridges in NovoPen series pens.

Max Headroom’s Retrofit Design

Max Headroom later developed a complete retrofit solution. Rather than modifying the original cartridge holder, he redesigned the holder itself as a 3D-printed replacement part. This eliminated the need for the separate threaded adapter.

Early versions still required a spacer to engage the clutch mechanism, but later iterations removed that requirement. With the latest design, users can simply replace the original NovoPen cartridge holder with Max Headroom’s 3D-printed version, allowing standard 3 mL cartridges to be used with NovoPen series pens.

Important Design Consideration

The main limitation of this design is material strength. Because the replacement holder is 3D printed, it is not as strong as the original injection-molded NovoPen cartridge holder. Injection molding produces a more durable part, so users should handle the 3D-printed holder with appropriate care.

Available from Daro Pep Accessories

Max Headroom’s cartridge holder design is available through Daro Pep Accessories and is included with all NovoPen series pens sold here.