Isoniazid

In Aotearoa New Zealand isoniazid is available as tablets (100mg).

• The dose of isoniazid is different for different people, depending on your body weight.
• The usual dose in adults is 300mg once daily.
• Always take isoniazid exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much to take, how often to take it and any special instructions.

• Timing of your dose: Swallow the tablets with a glass of water, on an empty stomach, at least 1 hour before or 2 hours after food. If you find it difficult to swallow the tablets, you can crush them before taking them. Don't take indigestion remedies, iron or calcium preparations within 2 hours of taking this medicine.
• Missed dose: If you forget to take your dose, take it as soon as you remember. But if it's nearly time for your next dose, take the next dose at the right time. Don't take extra doses to make up for a forgotten dose. If you often forget to take isoniazid, your tuberculosis may not be fully treated.
• Finish the course: You will usually be asked to take isoniazid every day. Continue to take the tablets regularly unless your doctor tells you to stop. This is because it's important for you to complete the treatment course so that the infection doesn't come back. If for any reason you stop taking the tablets (eg, if you think you may be developing side-effects – see below) then you must let your doctor know about it straightaway so that you can be given different treatment.

Here are some things to know when you're taking isoniazid. Other things may be important as well, so ask your healthcare provider what you should know about.

• Some foods may interact with isoniazid. Try to avoid eating foods containing histamine or tyramine (eg, cheese, fermented foods and sauces, soy sauce, yeast extract, some fish such as tuna, and red wine). Taking isoniazid with these foods may cause fast heart beat, palpitations, flushing, throbbing headache, dizziness, or sweating.
• Isoniazid can interact with some medicines, herbal supplements and rongoā Māori, so check with your doctor or pharmacist before starting isoniazid and before starting any new products.
• Limit alcohol while you are taking isoniazid. Alcohol can increase your chance of side effects such as problems with your liver.
• Tell your doctor if you are pregnant, planning to become pregnant, or breastfeeding.
• Tell your doctor if you have diabetes, epilepsy, HIV, problems with your kidneys or liver.

Like all medicines, isoniazid can cause side effects, although not everyone gets them. If you're concerned about any symptoms you think might be related to your medicine, talk to your healthcare provider. The following information offers some guidance but doesn't include all possible side effects.

Common side effects

Tell your healthcare provider if these side effects bother you.

• Nausea (feeling sick), vomiting (being sick) or a sore tummy.
• Fast heartbeat, palpitations, flushing, throbbing headache, dizziness, or sweating: These can occur if you are taking isoniazid with foods or drinks containing histamine or tyramine, eg, cheese, fermented foods and sauces, soy sauce, yeast extract, some fish such as tuna, and red wine. Avoid these foods. Let your healthcare provider know if these symptoms don’t go away.

Tell your healthcare provider immediately or phone Healthline free on 0800 611 116 if these occur

• Numbness or tingling feelings in your fingers and toes.
• Muscle weakness or ache.
• Feeling dizzy, slurred speech, unsteadiness.
• Skin rash or itching.
• Bruises or red and purple spots on your skin that you cannot explain.
• Signs of problems with your liver such as severe tummy pain, yellowing of the eyes and skin, dark urine.

Read more about medicines and side effects and reporting a reaction you think might be a side effect.