Us4597961 transdermal nicotine patent expiration?

Patent: US 4,597,961
Title: “Transdermal drug delivery system, especially for nicotine”
Grant date: November 10 1992

The U.S. patent system grants a standard term of 20 years from the earliest filing (priority) date unless a term extension is granted. For US 4,597,961 the earliest filing date was July 7 1989. Consequently the 20‑year term ran to:

Expiration date: July 7 2009

After that date the claims of the patent are in the public domain, meaning the technology can be used without licensing that particular patent. (Note that newer or related patents could still be in force, so always check for any later filings or extensions.)