My grandmother's new medications are causing her to have constipation frequently. What can I do to help?

Medications are Cardizem, Lasix and Seroquel and her doctor wants her to stay on them indefinitely. Her next appt is months away. She also has high blood pressure / congestive heart failure. I have tried giving her fruits & vegetables, more water, and stool softeners. How often can a stool softener be taken?