Mrs MacVicar, from Bridge of Weir, is reported to have claimed her son would have died had they not fled.
She said from an undisclosed location in Spain: "I believe the medication was killing Andrew. I couldn't stand by and watch. I'll never give up fighting for my son."
Mr MacVicar was taken into care two years ago. He has Asperger's syndrome and was also diagnosed as schizophrenic.
He is under a compulsory treatment order (CTO) to take a course of medication, and had been living at accommodation run by Renfrewshire Association for Mental Health in Paisley.
Mrs MacVicar said his medication has left him suicidal and has given him tremors and weight gain.
She said the couple have planned to visit hospitals in Spain and are "receiving support from medical professionals".
Mrs MacVicar believes her son has been misdiagnosed with schizophrenia.
Strathclyde Police confirmed they received a report from social services that Mr MacVicar had absconded and is believed to be in Spain.
A spokeswoman said CTOs are only applicable in the UK.
A spokeswoman for NHS Greater Glasgow and Clyde did not comment on the case.
Fiona Sinclair, convenor of Autism Right, said: "Our evidence suggest there are many people with autism trapped within the mental health system being given inappropriate treatment."