so far such amendment in drugs and cosmetics act is not within my knowledge.

All over the world, patent for drugs is issued only in molecular name.

It has to do with clarity. The whole problem with using trade names is the potential for confusion (lack of clarity), which may result in uneforceability of a patent. If a patent describes something with a trade name, does that mean the molecular species of the active ingredient, or the active ingredient, or the drug product, or the commercially available formulation including any extensions? What if the company changes the formulation?

On the other hand, using molecular formula is generally not ambiguous.

There is nothing wrong with using trade names in patents, as long as you are very, very careful that it is clear from the description what the trade name means.