Evergreening of patents by pharma companies

Pharma companies are offered protection on their patents for 20 years by the pharmaceutical regulators of each country
However, in order to make more money, they indulge in evergreening of patents

What is it?

They make few minor tweaks in the formulation of the drug - like replacing compound X with compound Y.
Even though the difference due to the tweak is trivial, the company claims it as breakthrough making outlandish claims like - 25% more effective now.
They file another patent for the tweak around a year before the current patent expires and ask for an extension
They do this in the hope of earning money from the drug for another 20 years - total of $20 + 19 = 39$ years.
If successful, this delays the entry of generic drugs into the market
According to a research, out of all the patents issued between 2005 and 2015, 78% were for extensions - evergreening.