You are making an assumption that the lower MW truly is safer, not just another colour.
If I patent a blue wonder hammer, then later try to extend the term by patenting a pink version with the claim that it strikes harder, we have an issue.
Could you extend the patent forever just by changing colours?
Did TEVA really make a better version, or just change the colour?
TEVA told the FDA that the product was the same wrt safety and efficacy.