View source

===Adverse effects===
For information on adverse effects of individual anti-TB drugs, please refer to the individual articles for each drug.

The relative incidence of major adverse effects has been carefully described:<!--
--><ref name="Yee2003">{{cite journal
 | author=Yee D ''et al.''
 | title=Incidence of serious side effects from first-line antituberculosis drugs among patients treated for active tuberculosis
 | journal=Am J Resp Crit Care Med
 | volume=167
 | issue=11
 | year=2003
 | pages=1472&ndash;7
 | url=http://ajrccm.atsjournals.org/cgi/content/full/167/11/1472
 | pmid=12569078
 | doi=10.1164/rccm.200206-626OC }}</ref>
* INH 0.49 per hundred patient months
* RMP 0.43
* EMB 0.07
* PZA 1.48
* All drugs 2.47
This works out to an 8.6% risk that any one patient will need to have his drug therapy changed during the course of standard short-course therapy (2HREZ/4HR). The people identified to be most at risk of major adverse side effects in this study were:
* age >60,
* females,
* HIV positive patients, and
* Asians.

It can be extremely difficult identifying which drug is responsible for which side effect, but the relative frequency of each is known.<!--
--><ref name="Ormerod1996">{{cite journal
 | author=Ormerod L. P., Horsfield N.
 | title=Frequency and type of reactions to antituberculosis drugs: observations in routine treatment
 | tournal=Tuber Lung Dis
 | volume=77
 | issue=1
 | year=1996
 | pages=37&ndash;42
 | pmid=8733412 }}</ref> The offending drugs are given in decreasing order of frequency:
* Thrombocytopaenia: RMP
* Neuropathy: INH
* Vertigo: STM
* Hepatitis: PZA, RMP, INH
* Rash: PZA, RMP, EMB

'''[[Thrombocytopaenia]]''' is only caused by RMP and no test dosing need be done. Regimens omitting RMP are discussed below. Please refer to the entry on [[rifampicin]] for further details.

The most frequent cause of '''[[peripheral neuropathy|neuropathy]]''' is INH. The peripheral neuropathy of INH is always a pure [[sensory neuropathy]] and finding a motor component to the peripheral neuropathy should always prompt a search for an alternative cause. Once a peripheral neuropathy has occurred, INH must be stopped and pyridoxine should be given at a dose of 50mg thrice daily. Simply adding high dose pyridoxine to the regimen once neuropathy has occurred will not stop the neuropathy from progressing. Patients at risk of peripheral neuropathy from other causes ([[diabetes mellitus]], [[alcoholism]], [[renal failure]], malnutrition, pregnancy, etc.) should all be given [[pyridoxine]]] 10mg daily at the start of treatment. Please refer to the entry on [[isoniazid]] for details on other neurological side effects of INH.

'''Rashes''' are most frequently due to PZA, but can occur with any of the TB drugs. Test dosing using the same regimen as detailed below for hepatitis may be necessary to determine which drug is responsible.

'''Itching''' RMP commonly causes itching without a rash in the first two weeks of treatment: treatment should not be stopped and the patient should be advised that the itch usually resolves on its own.  Short courses of sedative antihistamines such as [[chlorpheniramine]] may be useful in alleviating the itch.

'''Fever''' during treatment can be due to a number of causes. It can occur as a natural effect of tuberculosis (in which case it should resolve within three weeks of starting treatment). Fever can be a result of drug resistance (but in that case the organism must be resistant to two or more of the drugs). Fever may be due to a superadded infection or additional diagnosis (patients with TB are not exempt from getting influenza and other illnesses during the course of treatment). In a few patients, the fever is due to drug allergy. The clinician must also consider the possibility that the diagnosis of TB is wrong. If the patient has been on treatment for more than two weeks and if the fever had initially settled and then come back, it is reasonable to stop all TB medication for 72 hours. If the fever persists despite stopping all TB medication, then the fever is not due to the drugs. If the fever disappears off treatment, then the drugs need to be tested individually to determine the cause. The same scheme as is used for test dosing for drug-induced hepatitis (described below) may be used. The drug most frequently implicated as causing a drug fever is RMP: details are given in the entry on [[rifampicin]].

====Drug-induced hepatitis====
The single biggest problem with TB treatment is drug-induced hepatitis, which has a mortality rate of around 5%.<ref>{{cite journal
|author=Forget EJ, Menzies D
|title=Adverse reactions to first-line antituberculosis drugs
|journal=Expert Opin Drug Saf
|year=2006
|volume=5
|issue=2
|pages=231&ndash;49
|pmid=16503745
|doi=10.1517/14740338.5.2.231}}</ref>
Three drugs can induce hepatitis: PZA, INH and RMP (in decreasing order of frequency).{{ref|Ormerod1996}}<ref name="Steel1991">{{cite journal
 |author=Steel M. A., Burk R. F., DesPrez R. M.
 |title=Toxic hepatitis with isoniazid and rifampin: a meta-analysis
 |journal=Chest
 |year=1991
 |volume=99
 |issue=2
 |pages=465–471
 |pmid=1824929
 |doi=10.1378/chest.99.2.465}}</ref> It is not possible to distinguish between these three causes based purely on signs and symptoms. Test dosing must be carried out to determine which drug is responsible (this is discussed in detail below).

[[Liver function tests]] (LFTs) should be checked at the start of treatment, but, if normal, need not be checked again; the patient need only be warned of the symptoms of hepatitis. Some clinicians insist on regular monitoring of LFT's while on treatment, and in this instance, tests need only be done two weeks after starting treatment and then every two months thereafter, unless any problems are detected.

Elevations in bilirubin must be expected with RMP treatment (RMP blocks bilirubin excretion) and usually resolve after 10 days (liver enzyme production increases to compensate). Isolated elevations in bilirubin can be safely ignored.

Elevations in liver transaminases ([[alanine transaminase|ALT]] and [[aspartate transaminase|AST]]) are common in the first three weeks of treatment. If the patient is asymptomatic and the elevation is not excessive then no action need be taken; some experts suggest a cut-off of four times the upper limit of normal, but there is no evidence to support this particular number over and above any other number. Some experts consider that treatment should only be stopped if jaundice becomes clinically evident.

If clinically significant hepatitis occurs while on TB treatment, then all the drugs should be stopped until the liver transaminases return to normal. If the patient is so ill that TB treatment cannot be stopped, then STM and EMB should be given until the liver transaminases return to normal (these two drugs are not associated with hepatitis).

Fulminant hepatitis can occur in the course of TB treatment, but is fortunately rare; emergency liver transplantation may be necessary and deaths do occur.

====Test dosing for drug-induced hepatitis====
Drugs should be re-introduced individually. This cannot be done in an outpatient setting, and must be done under close observation. A nurse must be present to take patient's pulse and blood pressure at 15 minute intervals for a minimum of four hours after each test dose is given (most problems will occur within six hours of test dosing, if they are going to occur). Patients can become very suddenly unwell and access to intensive care facilities must be available. The drugs should be given in this order:
* Day 1: INH at 1/3 or 1/4 dose
* Day 2: INH at 1/2 dose
* Day 3: INH at full dose
* Day 4: RMP at 1/3 or 1/4 dose
* Day 5: RMP at 1/2 dose
* Day 6: RMP at full dose
* Day 7: EMB at 1/3 or 1/4 dose
* Day 8: EMB at 1/2 dose
* Day 9: EMB at full dose
No more than one test dose per day should be given, and all other drugs should be stopped while test dosing is being done. So on day 4, for example, the patient only receives RMP and no other drugs are given. If the patient completes the nine days of test dosing, then it is reasonable to assume that PZA has caused the hepatitis and no PZA test dosing need be done.

The reason for using the order for testing drugs is because the two most important drugs for treating TB are INH and RMP, so these are tested first: PZA is the most likely drug to cause hepatitis and is also the drug that can be most easily omitted. EMB is useful when the sensitivity pattern of the TB organism are not known and can be omitted if the organism is known to be sensitive to INH. Regimens omitting each of the standard drugs are listed below.

The order in which the drugs are tested can be varied according to the following considerations:
# The most useful drugs (INH and RMP) should be tested first, because the absence of these drugs from a treatment regimen severely impairs its efficacy.
# The drugs most likely to be causing the reaction should be tested as late as possible (and possibly need not be tested at all). This avoids [[Challenge-dechallenge-rechallenge|rechallenging]] patients with a drug to which they have already had a (possibly) dangerous adverse reaction.

A similar scheme may be used for other adverse effects (such as fever and rash), using similar principles.