The formulation sold under the brand name Methadose (flavored liquid suspension for oral dosing, commonly used for maintenance purposes) should not be injected either.Information leaflets included in packs of UK methadone tablets state that the tablets are for oral use only and that use by any other route can cause serious harm. In addition to this warning, additives have now been included in the tablet formulation to make the use of them by the IV route more difficult. Chemistry
Detection in biological fluids
Methadone and its major metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), are often measured in urine as part of a drug abuse testing program, in plasma or serum to confirm a diagnosis of poisoning in hospitalized victims, or in whole blood to assist in a forensic investigation of a traffic or other criminal violation or a case of sudden death. Methadone usage history is considered in interpreting the results as a chronic user can develop tolerance to doses that would incapacitate an opioid-naïve individual. Chronic users often have high methadone and EDDP baseline values. Conformation
The protonated form of methadone takes on an extended conformation, while the free base is more compact. In particular, it was found that there is an interaction between the tertiary amine and the carbonyl carbon of the ketone function (R3N ••• >C=O) that limits the molecules conformation freedom, though the distance (291 pm by X-ray) is far too long to represent a true chemical bond.
While phenazopyridine has never been shown to cause cancer in humans, evidence from animal models suggests that it is potentially carcinogenic. Pregnancy
This medication is pregnancy category B. This means that the medication has shown no adverse events in animal models, but no human trials have been conducted. It is not known if phenazopyridine is excreted in breast milk. Pharmacokinetics
The full pharmacokinetic properties of phenazopyridine have not been determined. It has mostly been studied in animal models, but they may not be very representative of humans. Rat models have shown its half-life to be 7.35 hours, and 40% is metabolized hepatically (by the liver). Mechanism of action
Phenazopyridines mechanism of action is not well known, and only basic information on its interaction with the body is available. It is known that the chemical has a direct topical analgesic effect on the mucosa lining of the urinary tract. It is rapidly excreted by the kidneys directly into the urine. Hydroxylation is the major form of metabolism in humans, and the azo bond is usually not cleaved. On the order of 65% of an oral dose will be secreted directly into the urine chemically unchanged. Brand names
In addition to its generic form, phenazopyridine is distributed under the following brand names:

References
External links
Information about phenazopyridine from the US National Library of Medicine
Interstitial Cystitis Association
American Urological Association