Heartworm and tick and flea protocols

I have two elderly dogs, one 15 and one somewhere between 16 and 18. The former is taking Denamarin for her liver (she weighs in at 5.2 lbs) and the latter is taking enacard for his heart murmur (he weighs in at about 14 pounds).

It seems that each month when I give them their heartworm meds, they either vomit or have diarrhea. In a day or so they are back to normal.

As for the tick and flea meds, I have been very reluctant to administer either.

I have spoken to two vets, both of whom have treated my dogs. One says continue with the meds and the other says to discontinue due to their age and medical conditions.

I would appreciate input from the forum.

Thank you,
Melissa, Huberta and Woodrow