Unless they include one of the unstable carrier oils discussed previously, most will endure at least two years before degrading. Some can even endure up to 15 years without losing their efficacy. To be safe, many experts recommend replenishing essential oils every three years. This can be done by purchasing more drops or by using them in higher concentrations. Essential oil companies typically specify how long you can expect a bottle to last. If it says three years, then that's about all you need to know. However, if it's a concentration rather than an exact amount, such as 10% v/v, that means one bottle could last for several years depending on how much you use it. A little goes a long way when using essential oils. There are various factors that can affect how long your oils will last including quality and brand, how much you use them, and whether they are diluted with alcohol or not. Oils that are distilled twice become less potent than those that are only distilled once. Essential oils are often blended by manufacturers according to demand for different effects. For example, if they think you might want a soothing effect but also want to get off of work, they may blend in some peppermint with the help of alcohol (usually vodka) to make the painkiller go further. These blends are usually labeled as having "extended duration" or "long-lasting".