A number of adverse drug reactions are possible from the use of antitubercular drugs. If cause is identifiable then it becomes easy to tackle the issue. Then the offending drug is withdrawn. However, if a reaction occurs but its nature does not single out a particular drug, the situation is little more difficult.
Then only the offending drug or drugs is cautious rechallenge.
In rechallenge, all the drugs are withdrawn and one by one they are reintroduced starting with the one least likely to be responsible for the symptoms.
Rechallenge is started with one drug in a small challenge dose, which is increased stepwise to full therapeutic dose over a few days.
This procedure is repeated, with one drug added at a time.
If the reaction recurs, the offending drug has been identified, it must be withdrawn.
Combination tablets are not suitable for this purpose and rechallenge should not be attempted.
Any reaction to thioacetazone, even if it is simple itching, should prompt immediate withdrawal of the drug and rechallenge should not be attempted with this drug.
Treatment may be continued by replacing the offending drug with a suitable alternative, or with a reduced number of drugs if none is suitable.
Specialist advice may be sought. It is also noteworthy that the resumed regimen is considered to be a new start to the treatment. This prolongs the duration of therapy but, on the other hand, reduces the chance of recurrence.
For example
Most antitubercular drugs, particularly pyrazinamide, rifampicin, and isoniazid, can cause hepatotoxicity, while ethambutol isseldom responsible. If a patient develops hepatitis, and no other cause is likely, drug-induced hepatitis must be presumed and the drugs stopped.
Once the hepatitis has resolved, the same regimen may be cautiously reintroduced. If the hepatitis has been severe, then it is probably safer to avoid pyrazinamide, and possibly also rifampicin, altogether.
An alternative regimen in such patients can be a 2-month initial phase of daily isoniazid, ethambutol and streptomycin followed by a 10-month continuation phase of isoniazid plus ethambutol.
A severely ill tuberculosis patient with drug-induced hepatitis may die without antiTB treatment. In this case the patient may be treated with the two least hepatotoxic drugs, namely streptomycin and ethambutol instead of interrupting TB treatment. Isoniazid may be cautiously reintroduced after the hepatitis has resolved.
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