Substitutes:

• Rye Whiskey: Bourbon can be a great substitute if you're out of rye whiskey, giving the drink a slightly sweeter profile.
• Fresh Lemon Juice: You can use bottled lemon juice in a pinch, but the flavor will be less bright and fresh. Another option is to use fresh lime juice for a slightly different citrus kick.
• Simple Syrup: If you don't have any simple syrup, you can easily make some at home by mixing together equal parts of sugar (any kind you like) and hot water. Stir well to dissolve and let it cool down before using it. In the refrigerator, this homemade simple syrup will last for about a month. Honey syrup (2 parts honey to 1 part warm water) or agave nectar can be used as natural sweeteners if you prefer not to use simple syrup.
• Absinthe: Pernod or any other pastis can be used as a substitute for Absinthe. They both have that strong anise flavor that is key to this cocktail.
• Egg White: Both fresh and pasteurized eggs can be used to prepare this cocktail. For those with egg allergies, aquafaba (chickpea water) is a fantastic vegan substitute with the same frothy texture.

Making it Non-Alcoholic:

• Rye Whiskey Substitute: Use a non-alcoholic rye whiskey substitute; these are available and do a good job mimicking whiskey's flavor. If you’re feeling crafty, you can also make your own rye whiskey substitute by infusing together rooibos tea and a bunch of spices of your liking (vanilla and cinnamon are always a good way to start). Mix all the ingredients together, add hot water, cover and let it infuse for at least 3h (the stronger the better, so if you have time let it infuse overnight. In that case, you can either let it rest in the fridge once the liquid has cooled down). Give it a taste, adjust the spice level if necessary and strain it once you’re happy with the result. Stored properly (in an airtight container in the fridge), this “tea whiskey” can be kept for up to a week.
• Absinthe Substitute: Opt for a non-alcoholic anise extract as a substitute for the absinthe. Also, note that three dashes of absinthe in an otherwise alcohol-free drink won’t raise the overall ABV of the drink: the drink will indeed chemically contain a drop of alcohol, but won’t make you intoxicated at all. Depending on your reason for not drinking alcohol and your degree of tolerance on the topic, you may decide to simply leave the absinthe out or to use it anyway when preparing your drink - both ways will work just fine!
• Proceed As Usual: Use this non-alcoholic substitute instead of the regular whiskey, follow the original steps, and enjoy a non-alcoholic version.

Making it Vegan:

• Egg White Substitute: Use aquafaba (the liquid from a can of chickpeas) instead of egg white to make this cocktail vegan-friendly.You can use ¾ ounce of aquafaba to replace one egg white. Follow all the steps, as aquafaba also needs to be shaken twice to make a proper foam.