Interaction between Simvastatin and Gemfibrozil
Major Synergy

ID DDInter1675 and DDInter812
Interaction Severe myopathy and rhabdomyolysis have been reported during concomitant use of HMG-CoA reductase inhibitors and fibric acid derivatives, especially gemfibrozil. Gemfibrozil has been reported to significantly increase the plasma concentrations of some HMG-CoA reductase inhibitors and/or their active metabolites, including lovastatin, simvastatin, pravastatin, cerivastatin, and rosuvastatin (but not fluvastatin). High levels of HMG-CoA reductase inhibitory activity in plasma is associated with an increased risk of musculoskeletal toxicity. Myopathy manifested as muscle pain and/or weakness associated with grossly elevated creatine kinase exceeding ten times the upper limit of normal has been reported occasionally. Rhabdomyolysis has also occurred rarely, which may be accompanied by acute renal failure secondary to myoglobinuria and may result in death. Other fibrates have not been shown to significantly affect the pharmacokinetics of HMG-CoA reductase inhibitors. However, the use of fibrates alone has also been associated with development of myopathy, thus a pharmacodynamic interaction could conceivably occur.
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Management Concomitant use of simvastatin and gemfibrozil is considered contraindicated by the manufacturer of simvastatin.
References
Alternative for Simvastatin A10B
Lixisenatide
Exenatide
Semaglutide
Dapagliflozin
Troglitazone
Sitagliptin
Alogliptin
Dulaglutide
Liraglutide
Miglitol
Ertugliflozin
More
Alternative for Gemfibrozil C10A
Probucol
Colesevelam
Dextrothyroxine
Simvastatin

Potential Metabolism Interactions

Substrate-Substrate Interaction：If more than one drug is metabolized by the same CYP, it is possible that its metabolism is inhibited because of the competition between the drugs. That means, it can be useful to lower the dosage of the drugs in the drug-cocktail because they remain longer in the organism than in monotherapy.
Inhibitor-Inhibitor Interaction：Combining two or more inhibitors of one CYP, should be compensated by lowering the dosage of these drugs because the metabolism is reduced and the drugs remain longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.
Inhibitor-Substrate Interaction：Combining drugs that have inhibitory effect and are substrates of one particular CYP, should be compensated by lowering the dosage. They rest longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.