This is because unlike the drugs in most other big dollar industries, insulin is not a chemical compound, but a biological compound.
No laws exist to deal with the issue of biological patents and because of this most companies found the existing insulin patents too hard to challenge.
The first real change came when >Sanofi Aventis managed to grab a chunk of the market in 2000 with the introduction of its novel once-daily long acting human insulin, Lantus, achieving worldwide revenue of $290 m in the first two years.
The next change came more recently when Novo finally won the bitter tug-of-war and broke free from Lilly after launching a series of novel insulin analogs that proved popular with patients.
The continued rollout of its insulin analogs, coupled with the increase of its US-based sales force to 600 reps finally knocked Lilly from the top spot and Novo Nordisk moved into a US leadership position for the first time in late 2005.
With the approval of its new long-acting insulin analogue, Levemir, in June last year, Novo Nordisk also became the first company with a full range of insulin analogues in the US.
Shortly after, Eli Lilly admitted defeat and announced the discontinuation of its older-style Iletin II Pork Insulin, Humulin U Ultralente and Humulin L Lente insulin products.
"Over the past few decades there has been a great deal of innovation in insulin therapy, such as rapid-acting analogs, analog mixtures, basal analogs and other human insulin products and formulations," said Dr. Scott Jacober, medical advisor, diabetes care, Eli Lilly.
"As a result, we've seen a significant and steady decline in usage of the products we're discontinuing as doctors continue to move their patients towards newer therapies."
However, Novo may have won the battle for now but its reign may only be short lived, as the anticipated launch of non-injectable insulin in the next few years is bound to shake up this multibillion-dollar-a-year market once again.
This is because an alternative to injections has been long-awaited by the 40-50 million Type 2 and 10 million type 1 diabetics worldwide who need insulin therapy but dream of one day abandoning their needles.
More importantly, the thought of having to self-inject several times a day results in many diabetics not taking their insulin treatment regularly, leaving them at risk of serious health complications such as blindness, cardiovascular and circulatory problems and kidney failure.
Therefore, if successful non-injectable insulin products are launched, analysts predict sales could exceed $5 bn by the end of this decade.
In a pre-emptive move to capture the future insulin market and prevent current market share erosion, Novo, Lilly and Sanofi/Pfizer have been working with biotech and drug delivery companies for years to develop various forms of insulin that is inhaled by the lungs and product launches are now imminent.
As Novo's reign comes under siege, the once sluggish insulin industry is poised to explode in a flurry of activity, as Lilly attempts to claw its way back to the top and Sanofi/Pfizer looks to blow them both out of the water with Exubera, the product likely to hit Pharmacy shelves by the middle of this year.