Prodrugs

Sometimes drugs are formulated deliberately so they will break down inside the body to form the active drug, these are called prodrug

Prodrug

A prodrug is a pharmacological substance administered in an inactive form. Once administered, the prodrug is metabolised in vivo into an active metabolite, a process termed bioactivation. The rationale behind the use of a prodrug is generally for absorption, distribution, metabolism, and...

s. The reason for this may be because the drug is more stable during manufacture and storage as the prodrug form, or because the prodrug is better absorbed by the body or has superior pharmacokinetics (e.g., lisdexamphetamine).

Metabolites of drugs

Another kind of active metabolite is when a drug is broken down by the body into a modified form which continues to produce effects in the body. Usually these effects are similar to those of the parent drug but weaker, although they can still be significant (see e.g. 11-hydroxy-THC

11-Hydroxy-THC

11-Hydroxy-Δ9-tetrahydrocannabinol, abbreviated as 11-OH-THC, is the main active metabolite of THC which is formed in the body after cannabis consumption. 11-Hydroxy-THC has been shown to be active in its own right, but the effects produced are not necessarily identical to those of THC...

, morphine-6-glucuronide

Morphine-6-glucuronide

Morphine-6-glucuronide is a major active metabolite of morphine, and as such is the molecule responsible for much of the pain-relieving effects of morphine . M6G is formed from morphine by the enzyme UDP-Glucuronosyltransferase-2B7...

). Certain drugs, such as codeine

Codeine

Codeine or 3-methylmorphine is an opiate used for its analgesic, antitussive, and antidiarrheal properties...

 and tramadol

Tramadol

Tramadol hydrochloride is a centrally acting synthetic opioid analgesic used in treating moderate pain. The drug has a wide range of applications, including treatment for restless legs syndrome and fibromyalgia...

 have metabolites that are stronger than the parent drug (morphine

Morphine

Morphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...

 and O-desmethyltramadol

O-Desmethyltramadol

O-Desmethyltramadol is an opioid analgesic and the main active metabolite of tramadol.-O-Desmethyltramadol is the most important metabolite of tramadol produced in the liver after tramadol is consumed...

 respectively) and in these cases the metabolite may be responsible for much of the therapeutic action of the parent drug. Sometimes however metabolites may produce toxic effects and patients must be monitored carefully to ensure they do not build up in the body. This is an issue with some well known drugs such as pethidine

Pethidine

Pethidine or meperidine Pethidine (INN) or meperidine (USAN) Pethidine (INN) or meperidine (USAN) (commonly referred to as Demerol but also referred to as: isonipecaine; lidol; pethanol; piridosal; Algil; Alodan; Centralgin; Dispadol; Dolantin; Mialgin (in Indonesia); Petidin Dolargan (in Poland);...

 (meperidine) and dextropropoxyphene

Dextropropoxyphene

Dextropropoxyphene, manufactured by Eli Lilly and Company, is an analgesic in the opioid category. It is intended to treat mild pain and has, in addition, anti-tussive and local anesthetic effects. It has been taken off the market in Europe and the US due to concerns of fatal overdoses and...

.