Bitter Pill by Fern Michaels

The Sisterhood: a group of women from all walks of life bound by friendship and a quest for justice. Armed with vast resources, top-notch expertise, and a loyal network of allies around the globe, the Sisterhood will not rest until every wrong is made right.

For the Sisterhood, every mission matters. But there's a special satisfaction that comes with helping a friend in need, especially when it's someone as dear as Myra Rutledge's childhood friend Charlotte. After she receives a worrying letter, Myra quickly arranges for Charlotte to visit, hoping to restore her peace of mind. She learns that Charlotte has spent tens of thousands of dollars on "natural herbal remedies" as part of an innovative program to boost longevity. But far from improving her health, the medications seem to be destroying it.

Myra becomes suspicious of the trio of doctors in charge of the program, especially once the Sisterhood's investigations reveal that one patient has died, and another lapsed into a coma. While those in their care suffer, the three doctors--located in London, Aspen, and New York--all enjoy indulgent lifestyles and extravagant toys. But the passions and obsessions paid for by their selfish scheme may be the very means to get revenge. Justice, after all, is always the best medicine--and no one dispenses it better than the Sisterhood...