What is the amount of medicine on a teaspoon measured in?

Approximately 6 ml = 1Teaspoon

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A teaspoon is 1/6 ounce. 3 teaspoons = 1 tablespoon, 2 tablespoons = 1 ounce.

an ounce (US) is ~29.6 ml so a teaspoon would be 4.9333...ml

an ounce (Imp) is 28.35 ml (the volume of one ounce[weight] of water) so a teaspoon would be 4.725 ml

Unless your teaspoon is defined as a measure equal to 1/96 of a pint (imperial measure) which would then make it 5.9 ml a much larger measure. You may be better off keeping your measures in milliliters or cc.