Is it tofacitinib or tofactinab or tasocitinib? Why are some drug names so difficult to spell and/or pronounce?

It is tofacitinib.

As an experimental drug, the compound was known as CP-690,550 or tasocitinib. However, tasocitinib was rejected as an International Nonproprietary Name (INN) due to the potential for confusion with other drug names, and tofacitinib was adopted instead. The naming of drugs is difficult because the names must be unique to avoid confusion with other similar sounding/looking drug names. This in a large part explains why many drugs have seemingly unnecessarily complicated and tongue-twisting names.

As tofacitinib was known as CP-690,550 or tasocitinib during its early development, you will still find preclinical research papers and early clinical trial papers referring to the compound by these names. As preclinical researchers first knew the compound as CP-690,550 or tasocitinib, some suppliers continue to sell the compound for preclinical research under these old names. For this reason, the old names still persist in some more recent papers.

Tofacitinab is a misspelling that you will see in some published papers.