Solution-processed gain media have great technological potential as lasers due to their ease of integration with on-chip photonics, scalability and tuneable optoelectronic properties. Currently, the spectral coverage of solution-processed lasers extends from visible up to telecom wavelengths in the short-wave infrared (SWIR) (<1650 nm). Here, the optical gain in the extended SWIR from 1600 nm to 2500 nm is demonstrated, using PbSbased colloidal quantum dots (CQDs). This spectral region has many applications such as in LIDAR, biological imaging and environmental monitoring and is currently served by exotic, costly materials with limitedscalability. Using the CQDs in distributed feedback laser cavities, lasing with emission tuned between 2150 nm and 2500 nm is reported. We show that due to the increased absorption cross-section of larger CQDs, the optical gain threshold is reduced by a factor of 36 compared to smaller-sized CQDs, reaching an amplified spontaneous emission (ASE) threshold down to 42 J cm-2. Furthermore, gain and lasing under nanosecond excitation are demonstrated for the first time from PbS CQDs and use transient absorption spectroscopy data to model nanosecond gain thresholds. This paves the way for realizing compact and practical CQD infrared lasers and potentially toward electrically driven laser diodes.