To the Editor: Isoniazid is used extensively for the treatment of active and latent tuberculosis (TB). It is generally well tolerated by children, and hypersensitivity reactions resulting in skin rash and requiring cessation of treatment are rarely reported in this age group.1,2

We report a case of isoniazid hypersensitivity in a 21-month-old boy potentially exposed to TB in a childcare setting. He was one of over 80 children screened after contact with a childcare worker who showed a positive smear result. His initial tuberculin skin test (TST) was negative and, in line with New South Wales guidelines,3 he was commenced on isoniazid 150 mg daily (10mg/kg/day) while awaiting a repeat TST.

After 3 days of treatment, he developed a small number of round vesicular lesions on his tongue. They were associated with mild discomfort but his appetite was not affected. There were three small maculopapular lesions on his legs and back that reportedly looked like mosquito bites before blistering. The child remained afebrile and was systemically well.

The family general practitioner considered that this presentation was possibly an allergic reaction and isoniazid was discontinued. Population health staff were consulted, and the risks and benefits of further isoniazid treatment were discussed with paediatric TB specialists. It was recommended that, after the rash had resolved, isoniazid be reintroduced at half the dosage and with close supervision.

Two days after isoniazid 75 mg daily was recommenced, the rash recurred. The child’s mother described lesions appearing as “burns all over his tongue” and reported further sores around his lips and six welt-like lesions on his legs. Isoniazid was immediately discontinued, the skin lesions resolved within 5 days and no further antituberculous therapy was administered. His repeat TST 12 weeks after the initial test was negative and he remains well.

No other potential triggers for a hypersensitivity reaction were identified. In particular, no other medications were administered during this period or for the week before commencing isoniazid.

According to his mother, the child had experienced a similar reaction within 1 hour of a single dose of ibuprofen when he was 8 months old. Several tongue blisters were accompanied by a generalised fine maculopapular rash lasting several days.

We concluded that the child most likely had a hypersensitivity reaction to isoniazid that required discontinuation of treatment. We reported this to the Therapeutic Goods Administration, which advised that it had received seven other reports since 1991 of suspected hypersensitivity, but none were for children under 10 years of age.