If it's a foster child and the court has ordered such visits, it's probably impossible to get around it without going back to court or being held in contempt. You, as a foster parent, don't necessarily have the right to make that decision. Especially if you have no proof that the mother is using during the visits.
If she's being drug tested, I'm assuming that her caseworker gets the results. That might be the place to start.
For me it's a non-negotiable point. If a birth mom or birth family has a history of drug use and continues to use/test positive, I would revoke visitation. Even if she/they acted appropriately during visits, actions would/could be unpredicatable.
We are coming from a place that at this time, most drug usage on the birth mom's part is something we've included in our parameters for "matches".
I'm going to have to say at this point, I don't believe I would cancel visits so long as my child was not being affected, in danger, emotionally disturbed as a result, etc. If I ever came to a point when I felt my child's health, well-being, and security were threatened, that would not be the case. But I think having the connection to her bio family is THAT important... I just don't see how holding visitation rights as a "carrot on a stick" for a birthmom is beneficial for anyone. This is part of the hard and not always fun part of OA... you have to learn that you cannot control the other party or be responsible for them. I love my daughter's mom to pieces, but I don't always agree with her choices. But she's not my daughter, you know?
Wanting or being open to an 'open adoption' does not equate to being open to situations in which the BM and potentially the baby are addicted to drugs or other substances, or have other medical/mental issues.
We are open to an 'open adoption' but we are not open to a BM who has abused drugs beyond 'occasional pot'.
At the end of the day, if we were matched with a BM in an open adoption who was not mentally or physically capable of having a positive relationship with OUR child then we as parents would have to intervene and potentially close off contact or physical contact with the BM - which would be our parental right. Being open does not mean you are in a 'shared custody' situation or that the BM can 'just change her mind' at any point. Most open adoptions stipulate at the min. that full names and addresses are shared - they can advance to physical contact and visits but visitation is rarely legally defined or structured in adoption.
So yes, as a parent I would probably not schedule more dates/visits and explain that to the BM. Since there is generally not 'regularly' scheduled visits (as in custody visitation), I guess I wouldn't use the phrase 'cancel visits' (I know semantics but it matters, at least to me that it is clear that we may allow a play date/lunch/birthday party visit but not structured visitation which implies some legal rights of the BM to visitation).