Thanks for your concern about your health! The herbal supplements you are taking, like Chondroitin, Quercetin, Glucosamine may be natural, but they are just as powerful as regular medicines, and have many side effects by themselves, and can and will interact with the type of food that you eat and other medicines you are currently taking, like Sertraline 25mg. This is a medication quite like Prozac, and is given in cases where there might be anxiety, or repeated unwanted thoughts, or states of sadness ( depression ). Your GP is right when he says that the herbal supplements may cause problems with Sertraline. I kindly suggest that you speak to your GP outright about how you can accomodate quercetin herbal supplement with regular medication because quercetin is good for health, if and only if it is taken in the right amounts, under guidance. Kindly verbalize these problems with your GP. Please feel free to post any further queries.