Some meds just don’t blend well. A recent and surprising mismatch: the HIV drug Kaletra (lopinavir/ritonavir) and the cholesterol-lowering Crestor (rosuvastatin). This past January, it was revealed that Kaletra can raise blood levels of Crestor by as much as two times, threatening to boost Crestor’s potential side effects. These can include muscle or stomach pain, weakness, constipation and nausea. Researchers are investigating to see whether Crestor can be taken at a lower dose with Kaletra.

Then there’s the HIV med Sustiva (efavirenz) and the anticonvulsant Dilantin (phenytoin). Mixing them can drop Sustiva levels in your blood, possibly keeping the drug from suppressing HIV—and risking illness and drug resistance.

Check all your meds for possible interactions by searching “Check My Meds” at