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In the past I have read a couple of times that one should not consume milk-based-products when taking certain medication for a certain amount of time before and after taking the product. To be a little bit more concrete, for certain types of Antibiotica this seems to be the case (Doxycyclin, etc.).

I'm of course completely fine with obeying that rule but as we're all humans and sometimes forget things, I wonder what I'm supposed to do if I accidentally drink a glass of milk and should take, let's say the Antibiotica, in the next hour. Should I run to my doctor? Or am I supposed to wait two/three hours and then take the medicine? Or should I just take it normally and hope that it works out?

I tried to look up the answer but besides some forum-posts there doesn't seem to be any kind of "official" statement (from a pharma company or something equivalent) about how to handle such a situation (or at least I wasn't able to find it).

Can someone maybe explain what the right reaction in such a case would be?

Doxycycline is a chelating agent. As such it will form a metallo-tetracycline complex in the gut which is basically not absorbed. So, you don't get the benefit of the tetracycline. So, you should take them 3 hours before or after meals. And if you have taken it with a glass of milk, the sensible thing to do is take it again after a few hours.

Tetracyclines have a high affinity to form chelates with polyvalent metallic cations such as Fe+++, Fe++, Al+++, Mg++ and Ca++. Many of these tetracycline-metal complexes are either insoluble or otherwise poorly absorbable from the gastro-intestinal tract. Milk and other dairy products, antacids containing polyvalent cations, as well as various iron salts ingested simultaneously with tetracycline derivatives, might interfere with their absorption by 50 to 90% or even more.

https://www.ncbi.nlm.nih.gov/pubmed/946598

Dietary restrictions on medication are for a variety of reasons:

Interactions that prevent absorption/action - this could include foods that somehow bind with the active ingredient of the drug and therefore limit it's action. The result could be similar to forgetting to take the medication at all.

Interactions that prevent enzymatic breakdown - these are common with certain liver enzymes that are involved in metabolism of drugs but can also be inhibited by foods. They could cause levels of the drug to build up to higher than expected levels because the active ingredient is not metabolized by the next dose.

Foods that cause similar effects to the drug - for example, drugs that cause drowsiness should typically not be taken with alcohol.

Because of the variety of reasons and uncertain level of severity for different drugs and different foods, all you should do is ask your doctor. You could also ask your pharmacist: it is also their job to know about interactions with drugs and they may be able to advise you. Your healthcare provider may also have something like a nurse's hotline that you can call in to, and they can either help you directly or guide you to other resources.