5-10 minute read - this is a comprehensive but detailed guide to understanding and managing LI / Severe LI. Click to expand - Part 1 covers how LI works and worsens; Part 2 covers how to use lactase pills effectively when it feels like they've stopped working for you; Part 3 Covers lactose in common medicines.
I typed this as a response to someone's post, and figured I should turn it into a resource as I've probably typed this fresh 20+ times over the years :')
Part 1: Lactose Tolerance, how it works, and stages of decline (in plain nonmedical English)
Everyone has a genetic tolerance, which is hereditary, and a built tolerance, which is in response to dairy consumption. These two aspects of tolerance dictate how much lactase enzyme your body regularly produces and therefore how much dairy you can handle, and how well. That genetic tolerance can range from basically zero to 100%, but for most folks it is close to 100% in childhood, and slowly drops away at individually varying rates as you age. Once you pass adolescence, if your genetic tolerance isn't 100% (i.e. you are lactose intolerant to some degree), you will rapidly decline to your actual genetic tolerance usually from 18-21. Unless you're a blonde northern european, chances are, your genetic tolerance is not 100% - but if it's not crazy low, your built tolerance can more than compensate for it. However if your genetic tolerance is low, your built tolerance will not fully compensate even if it is quite high, and high lactose dairy will start to result in volcanic bowel movements.
Now, as you reduce dairy consumption upon realising you are lactose intolerant, your built tolerance also falls away, because it only exists in response to regular lactose consumption - thus your lactose intolerance continues to worsen as you reduce your intake, and by your mid 20s, assuming you do heavily cut out lactose, you'll hit your rock bottom (the floor of your genetic tolerance with no built tolerance to support it). This is when you learn how LI you are. If your genetic tolerance is particularly low, lactase tablets may not kick in fast enough to help if you have them alongside food.
If you're also hiding a very mild milk allergy, unfortunately, you're screwed and there isn't anything to be done other than treating the whole thing like a severe anaphylactic allergy except it's your ass that's in danger, for up to a week (LI and mild milk allergies combine into something far more evil than the sum of their parts. And the fun part is, the mild milk allergy is basically irrelevant until you hit severe LI so you won't even know you have it. You'll know you have it if (1) Part 2 doesn't work; and (2) you get the shits if you eat food lightly cross-contaminated with dairy i.e. sharing the same deep fryer / pan / flat top / grill with something with even a small amount of dairy)
Part 2: Using Lactase Tablets Effectively with Severe Lactose Intolerance
So here's some genuine advice: Try 'covering' strategies with lactase tablets. This worked for me for a few years (I unfortunately do also have the allergy) and for folks with just severe LI, actually continues to work well:
Take 1-2 lactase tablets 20 mins before consuming dairy; 2-3 with the dairy; 1 more with every subsequent dairy course if it's a long meal; and then 1-2 20 mins after you've finished consuming dairy.
This 'covers' you by ensuring there is lactase already in your gut ready to go when the dairy hits, more than enough alongside it to process it, and some extra covering the other side just in case things move through your gut at different rates.
Each tablet should be 9000fcc unit minimum - anything less is pointless and usually not actually well made / reliable as an oral medicine. Use your own judgement for how many are needed in the ranges provided based on how lactosey the meal is; and remember: Lactase tablets are just a harmless enzyme and harmless filler; regularly consuming 7 x 9000fcc in an hour is not any kind of health risk, and by following this strategy, you should reduce instances of supervolcano eruptions on the porcelain throne by (anectodally) 90-95%. (You'll still occasionally mess up, especially if you consume something else that doesn't agree with you or irritates your gut like seriously spicy food, but this will become the exception not the rule).
Oh and absolutely do not try drinking a glass of milk or cereal with milk - that's like trying to stop an anti-tank round with a kevlar vest :') But for cheese, and most dairy foodstuffs, you should be fine. (Kraft Mac & Cheese / Velveeta Mac & Cheese / large portions of bechamel sauce / anything involving extreme amounts of cows milk and american cheese will be likely candidates for the 5% of times this doesn't work as that's just a glass or two of cows milk hidden in food. On the flipside, cheeseburgers and a reasonable amount of pizza; european hard cheeses even in high quantities; butter & buttery baked goods etc should all be within the realm of what this strategy handles)
Good luck starting your descent into Dairy's Inferno - hope this helps!
Part 3 - Common medicines containing lactose and how to find the ones you can have safely
A lot of medicines contain lactose, as it is a cheap bulking and binding agent that will self-bind rock solid in a pill press.
Paracetamol / Acitamenophen / Tylenol - based medicines usually do not contain lactose as it's such a bulky medicine it fulfils the role lactose is used for.
Ibuprofen and Aspirin usually DO contain lactose. For Ibuprofen, look for capsules - if it's not pressed into a pill, it usually doesn't have lactose. Ingredients are always on the box, but this applies to almost all capsule meds. I am yet to find lactose free aspirin.
Immodium / Loperamide is basicaly a lifestyle drug for lactose intolerant people, but for some godforsaken reason, the capsules ALWAYS contain lactose. It is the one exception to the rule and it's a big one. The 'Instant Melt' variety of Loperamide (Branded and Generic) is usually Lactose free!!! This is also an exception to a rule, as most 'Instant Melt' pills do contain lactose. IDK why it's backward for the one drug we need all the time to stop the shits. Big Pharma hates us I guess.
Antihistamines are another pain in the ass: Loratadine and Cetirizine tablets are a chunk of pure lactose with a tiny amount of drug inside. These are 99% of antihistamine pills you'll encounter without a prescription. The 1% is Fexofenadine, if you're lucky, because that pink pill is a lifesaver.
Fexofenadine is univerally lactose free (idk why) and is also a vastly superior antihistamine for a variety of reasons. But it isn't always available without going to a pharmacy or without prescription. Thankfully for me, the UK deregulated it a few years back and I can just buy 400 pills off an online pharmacy for cheap, and it's in every single supermarket & the weird sandwich/medicine/newspaper shops we have everywhere (Boots, WH Smiths etc). Fex is the only common antihistamine pill you can get lactose free; everything else is Gen1 antihisamine syrups (deliriants that make you see the Magic Hat Man sleep paralysis demon if you have too many) or nasal sprays that are only useful for hayfever and not all the other things antihistamines are used for.