Which forces are involved in holding the drugs to the active site of enzymes ?

Enzymes are basically the proteins which do the catalytic activity and there is a particular site present on the protein called active site which mainly bind to the upcoming substrate and helps on the formation of product.
The active site is in the shape of a three-dimensional cleft that is composed of amino acids (constituent of proteins) from different residues of the primary amino acid sequence. The amino acids that play a significant role in the binding specificity of the active site are usually not adjacent to each other in the primary structure, but form the active site as a result of folding in creating the tertiary structure. This active site region is relatively small compared to the rest of the enzyme.
The unique amino acids contained in an active site promote specific interactions that are necessary for proper binding and resulting catalysis. Enzyme specificity depends on the arrangement of atoms in the active site. Complementary shapes between enzyme and substrate(s) allow a greater amount of weak non-covalent interactions including electrostatic forces, Van der Waals forces, hydrogen bonding, and hydrophobic interactions. Specific amino acids also allow the formation of hydrogen bonds.