Chemical alteration of the drug in a living organism is called biotransformation.

The metabolism of a drug usually converts the lipid-soluble and unionized compounds into water-soluble and ionized compounds.

They are not reabsorbed in the renal tubules and are excreted. If the parent drug is highly polar (ionized), it may not get metabolized and is excreted as such.

Sites: Liver is the main site for drug metabolism: other sites are GI tract, kidney, lungs, blood, skin and placenta.

1.Active drug to inactive metabolite: This is the most common type of metabolic transformation

Phenobarbitone – Hydroxy Phenobarbitone

Phenytoin- P-Hydroxy Phenytoin

2.Active drug to active metabolite

Codeine – Morphine

Diazepam — Oxazepam

3. Inactive drug to active metabolite

Prednisone -Prednisolone

Levodopa- Dopamine