Customer reviews for Doctor's Best Acetyl L Carnitine with Biosint Carnitines 500 MG Capsules, 60 Ct Walmart.com L Carnitine: Benefits, Side Effects, And Dosage, Per Dietitians L Carnitine injection is recommended at 1 ampule every 2 3 days for best results. The L Carnitine is taken intravenously either by normal injection or using a butterfly needle. After taking this L Carnitine, you Did You Know? Some people report a fishy smell after supplementing with L carnitine. This happens because certain gut bacteria convert carnitine to trimethylamine, a malodorous compound found in rotting fish.