You can take over the counter Robitussin safely. The DM syrup does not have anything like aspirin or an NSAID drug in it. Coumadin will not interact with the DM Robitussin. When taking coumadin, what you want to watch out for over the counter are NSAIDS - Non-Steroidal Anti-Inflammatory Drugs. These drugs can cause blood thinning also. Check the ingredients list before you take over the counter drugs with Coumadin.