Presumably you mean osteoarthritis of the knee? You can safely take paracetamol at the stated dosage. Long term you can alleviate your knee pain by improving the strength of the surrounding muscles/tendons/ligaments and reducing the load on the knee joint.
Exercise your legs through squats, lunges, step ups, leg extensions (not working with any undue pain). Losing weight will ease the load on the joint and hence the pain.
The end game for unrelenting arthritic pain would be a knee replacement.