Legal Question in Criminal Law in California

Heroin and meth are not prescription drugs.

Even if your daughter took heroin and meth while she was pregnant, that would not mean she got them from you. I don't know what makes her mother-in-law think that you were the source -- or even that you gave your daughter prescription drugs, which is all she has accused you of doing.

The authorities would need good reason to think you did something wrong before they would take any action against you. Nothing in your question suggests they would have good reason. Of course, I don't have all the relevant facts. It's possible that there are reasons to suspect you that you haven't mentioned. You might not even know of those reasons yourself, especially if someone is willing to make false about you.

Sounds like somebody is making false accusations about you. If you know for a fact you did nothing wrong and somebody is implying that you did, you might even have the upper hand for suing in a defamation case.

If there is a problem with the authorities it is probably with CPS. If you have custody of the child or if the child is in your house a complaint of this type may bring CPS to your door.

Your daughter should be worried that custody of the baby will be taken away by CPS for 'unfit mother' and child abuse/neglect.