If you and your vet decide it's the Lantus causing the reaction, you can try Levemir. The way it works is similar - it's long lasting - but the mechanism is different. Here is some technical info from petdiabetes.wikia.com
Levemir is a clear solution created by genetically engineered yeast. It is analogous to human insulin, but includes a "fatty acyl side chain" that binds temporarily to albumin (found under the skin and in abundance in blood plasma) (98% albumin-bound[26]) and makes the molecule "cling" to the bigger albumin molecules for a while before releasing -- making the molecules slower to enter and exit the bloodstream, and slower to enter the cells.
At an ADA Symposium prior to Levemir's US approval in 2005, Novo Nordisk claimed that if NPH has a patient to patient variability of 60%, Lantus (insulin glargine) then has a 40% variability, and Levemir a 25% rate of variability. Unlike its long-acting rival Lantus, Levemir's time-delay action occurs in the bloodstream and at the target sites, not only at the subcutaneous injection site, and doesn't depend as much on the properties of the injection site or the suspension (which is Isophane, like NPH and other analog mixed insulins)[27][28].
Human insulin is altered to produce insulin detemir (Levemir) by omitting the amino acid Threonine at insulin B chain position #30 and attaching a C14 fatty acid chain to the amino acid at position #29, Lysine[29]. The combination of the omission of the threonine amino acid at position #30 on the B insulin chain and the addition of the fatty acid side chain at position B-#29 binds the insulin heavily to albumin[30].
