None at all.
You might think bottled lemon juice is an unacceptable substitute for the real deal, and it is true that once any kind of citrus juice comes in contact with oxygen, it starts losing its flavor and gets more acidic, which means it doesn't taste quite as lemony, limey or orangey as fresh would. And many commercial brands contain controversial additives. Still, using bottled lemon juice is better than none at all—provided you use a high-quality brand. Sabrina Sexton, a chef instructor at the Institute of Culinary Education in New York, uses Santa Cruz Organic Pure Lemon Juice or Nellie & Joe's Key West Lemon Juice for cocktails. If you're caught without a good quality bottle of lemon juice, consider adding a splash of orange juice or even vinegar (not for drinks, though), which will add the burst of acidity and freshness lemon juice usually brings to a dish.