Warfarin and osteoarthritis (OA) pain relief medicines

Written by Linda Caddick Registered Pharmacist and Arthritis Assist team member 

We are often asked about medicine interactions, especially when people are taking warfarin. 

Warfarin is a blood-thinning medicine that helps prevent blood clots. It is well known for interacting with other medicines, and even some foods. Because of this, it’s important to check what is safe if you are also managing osteoarthritis (OA) pain. 

Our pharmacist has reviewed how common OA pain medicines may interact with warfarin, based on guidance from the New Zealand Formulary (NZF). 

Tricyclic (e.g. amitriptyline, nortriptyline) 
These are often safe to use, but they may increase variability in blood clotting times.  

• Tricyclic (e.g. amitriptyline, nortriptyline) 
  These are often safe to use, but they may increase variability in blood clotting times.  

Gabapentin and pregabalin  

• No known interactions with warfarin.  

Carbamazepine  

• Not recommended. 
• It can significantly reduce the effectiveness of warfarin, increasing the risk of blood clots.  

Oral steroids (e.g. prednisone)  

• Use with caution and under medical guidance. 
• Higher doses can increase the effects of warfarin, raising INR levels and bleeding risk.  

Codeine

• No known interaction with warfarin. 
• However, it is not usually effective for arthritis pain.  

Nefopam  

• No known interaction with warfarin. 
• Also not commonly effective for arthritis pain.  

Topical NSAIDs (e.g. diclofenac/Voltaren gel)  

• The interaction risk is uncertain. 
• While oral NSAIDs are not recommended with warfarin, there has been at least one report of increased INR with topical use. 
• It’s best to check with your GP or pharmacist.  

Capsaicin cream (e.g. Zostrix)  

• No known interaction with warfarin. 

• Everyone responds differently to medicines. 
• Always talk to your GP, nurse, or pharmacist before starting, stopping, or changing any medicines. 
• Arthritis NZ does not prescribe or diagnose. Our role is to provide information and support so you can make informed decisions alongside your healthcare team. 
• Keep up with regular INR testing if you are taking warfarin.  

If you have questions about your medicines, side effects, or possible interactions, you’re welcome to get in touch with us. 

Our pharmacist Linda can help you understand your medicines and talk through any concerns you may have. 

Call our 0800 663 463 Arthritis Assist line or info@arthritis.org.nz