Should I dose Tylenol by weight or by age?

What the evidence says

Always dose by weight if you can. Health Canada is clear about this: the right amount of acetaminophen for your child is based on their weight, and age is only used as a backup when you don't know how much they weigh.

This matters because two kids the same age can be very different sizes. A study looking at children aged 2 to 3 years found that the standard age-based dose of 160 mg often didn't match what their bodies actually needed, since that dose was really only well matched to kids weighing about 11 kg. Some children in that age group weigh quite a bit more or less.

Canadian pharmacy researchers have also pointed out that mixing up different bottle strengths and relying on age instead of weight is a common reason parents accidentally give too little or too much medicine. Giving too little means the fever or pain doesn't get better. Giving too much, especially repeatedly, can strain the liver.

In practice: weigh your child (or use their last clinic weight), check the concentration printed on the bottle (they come in different strengths), and use a proper dosing syringe rather than a kitchen spoon. Canadian guidance suggests 10 to 15 mg per kilogram of body weight per dose, no more than five doses in 24 hours, spaced 4 to 6 hours apart.

If you genuinely don't know your child's weight, the age-based chart on the package is a reasonable fallback, but it's less precise. When in doubt, ask your pharmacist to walk you through the calculation for your child's exact weight.