Some psychiatry medicines give unbearable burning sensation (can't explain) in stomach which is not nausea.but i can cope up with the same medicine of some other brand. why is it so?

Possible: While there is little evidence that the generic medications are less effective or have more side effects than the brand name medicines, it is often seen in clinical practice that some people do feel different with different brands of the same medication. Discuss with your psychiatrist who will be able to help you switch to a medication that is safe and effective for you.

Same med,: Different formulation. This happens sometimes when a different formulation is used. Years ago i remember a new formulation of nortiptyline capsules causing this complaint in quite a number of patients. The capsule would cause mucosal irritation. I put one in the toilet, and it immediately stuck to the inside of the toilet bowel. I thought " would i really want that stuck to my stomach lining?".

Med side effect: Different brands/formulations of medications can have different coatings or "fillers" (the non-active part of the medicine) that someone might be sensitive to in certain cases, causing stomach upset/reflux (which might be described as burning). Taking medications with food often helps with such side effects. (most psychiatric medications can be taken with food.).

Depends on additive: If i understand correctly you experience such side effects only from certain generic brands of the same medication. This can occur due to the additives, coating of the pill. I would suggest keep track of the manufacturer and then only get that brand type. You can get your doctor then specify on the prescription that generic brand.