Colchicine is a substrate for both cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp). Extended-release diltiazem (diltiazem ER) is a potent inhibitor of both CYP3A4 and P-gp. This study will evaluate the effect of multiple doses of diltiazem ER on the pharmacokinetic profile of a single 0.6 mg dose of colchicine. A secondary objective is to evaluate the safety and tolerability of this regimen in healthy volunteers. All study subjects will be monitored for adverse events throughout the study period.