I've been informed that there are a variety of medications that can slow down the progression of neck arthritis. What are they & which fewest side eff?

All have side effect. Arthritis meaning inflammation to the joints can be treated a number of ways. Medication wise, otc such as tylenol (acetaminophen) is well tolerated if it helps. Others such as Ibuprofen does help as well. But of course side effects can occur. Please consult with your medical professional for further guidance on this chronic condition.
Depends on diagnosis. There are medications for inflammatory arthritis like methotrexate, biologics that can slow or stop inflammatory damage. There are no stoppers for mechanical wear and tear yet. Physical therapy, neck pillows, analgesics, exercises can help.