Thanks everyone for your kind advice.
We had a heart to heart and I told him that I would not stand for it anymore. He agreed that he loves our daughter and does not want her to come to any harm. He admitted that he never took me seriously as he thought I was just nagging for the sake of it. I provided him with stories of brain seizures and heart problems that I found during my research on the dangers of accidental ingestion of Ritalin by toddlers. He was as horrified as I was. We have now both agreed that if he wants his medication, he has to come home at lunchtime (5 min walk!) and get it from me. I have hidden it. In the meantime, I have ordered a combination lock medicine cabinet – so even if he finds it, he has to ask me to get it as only I will know the code. I hope that this will remedy the situation and thank you again for your thoughts, which were most helpful and insightful.