We've had this question before - if only I could remember the answer clearly. I seem to recall it was something about not using the fruit that were present at purchase because of some treatment the growers use, but that fruit produced in subsequent years would be fine to use. Hopefully someone else will recall more clearly. If they don't, use the Google box at the top of the GoY page, type in Lemon tree or Citrus tree and that should take you to a list of previous questions on this plant.

Bamboo is right. The present fruit may be sprayed with chemicals to encourage fruiting (you are more likely to buy the Lemon Tree if it had fruit on it). New Lemons are fine.

Many thanks for your replies. This is what I thought, but was good to hear it from others.