Yes, using a menstrual cup (such as DivaCup) is much better for the environment.
I located a thesis paper written by a student at Dalhousie University for her BSc in Environmental Science and Gender and Women Studies which compares the financial and environmental costs of various menstrual products. Here's the relevant part of the abstract:
The environmental externalities of product raw materials show that for a temporal scale of one unit and one cycle or longer, the products with the least environmental effect are the tampon without applicator and the reusable menstrual cup respectively.
And here are some of the quantitative results:
- It takes as much fossil fuel to produce one DivaCup as about 8 tampons.
- The break-even point in fossil fuel usage between generic tampons and the DivaCup is about one cycle.
- The DivaCup uses about 93% less fossil fuel over the course of one year.
- Similar results are observed for global warming potential, acidification, and eutrophication.
An article in CHOICE came to the same conclusion:
It's impossible to deny the profound environmental reasons for switching to the reusable cups and underpants. There's also a clear cost benefit over a number of years of use for the menstrual cup.
Using a menstrual cup (such as a DivaCup) for even just a couple months is an improvement over tampons and other products. If the DivaCup is consistently used over its entire lifecycle, the difference in environmental impact is huge.