You are right to be concerned. If we don't get a withdrawal agreement agreed by (extended time is now mid November if she asks for the extension otherwise it's end of October) we will end up crashing out at the end of March next year. It's the crashing out, the No Deal that is the really big issue. If we agree the withdrawal agreement then we will have an 18-20 month transition period and nothing much will really change except gradually so medicines will still be coming along with everything else for that transition period and during that period monumental amounts of work will be carried out to try to secure all our current agreements so that we avoid the sudden stop. The problem we have is TM is trying to pretend intransigence as strength. It's not. The EU have always said they will not split the 4 pillars of the union, freedom of movement, trade, services and capital. We signed up to that, the EU hasn't said anything different ever. But she's trying to make out they are being unreasonable. This is the block to a withdrawal agreement, which is the legal part of the separation. She's still banging on about Chequers deal, but it's dead in the water and always was because it requires the EU to break those 4 pillars.
So there is a very real current danger (over 75% I believe) of us crashing out and everything coming to a sudden stop in April 2019. Medicine will be an issue. I have this worry too, with thyroid meds as well. I am sensitive to brand types and can't use some of them at all so personally, I have decided that I will slowly build up a small stockpile of my own meds between now and March and try to have enough on standby for 4-6 months. If it becomes unnecessary then I have my supplies and can just use them up. I will have to buy them. The NHS isn't going to suddenly let me up my repeat prescriptions. The same goes for the expensive Androfem Australian testosterone. Australia isn't in the EU but our current trade agreement with Australia is through the EU, something many people struggle to understand, and so in the event of crashing out without a withdrawal agreement, we won't be able to trade with them either until a new deal can be arranged or WTO terms put in place.
Planning and preparing never goes amiss. If it ends up being unnecessary, at least you could sleep at night.