Can you subsitute lemon juice for lemon extract?

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Since lemon extract is much stronger, I would use 1/2 again as much of the lemon juice. However, if your recipe calls for water or milk in the mixture, cut that back by a tablespoon or two because you are adding in a little more liquid with the lemon juice than the recipe calls for.
I'd only use real lemon juice or Real Lemon in the green bottle, if you are using it to make a substitute buttermilk, you can use vinegar.  

Grated rind is better

Lemon extract is so much stronger than juice that it will be almost impossible to get a similar flavor. I would try substituting lemon juice for ALL the liquid - water or milk - but you still may not get the same lemony flavor. A better bet would be to use the grated rind of 1 or 2 lemons.  

Beware Acidity

Lemon extract does not contain acid. If the recipe that calls for extract has something in it that would react with acid (ie. baking soda) juice would not be a suitable substitute.
you can take lemon juice concentrate and and boil it down some with a bit of sugar and you get anice substitute for extract.
Correct Answer

Lemon extract is made from lemon oil and alcohol. Lemon oil comes from the lemon zest (the yellow part of the peel). Therefore lemon zest would be the substitute for the lemon extract. Do not use lemon juice, it will make the dish lemony but very tart.  

More juice

well i perfer lemon juice. But yes you can use lemon extract but if you use;use half of what they call for for lemon juice. And that's b/c lemon extract is very strong..