WildCat posting this! - With his permission and because it is in interests of topic!
Impossible to predict how any medication will affect each individual as metabolism, general health, predisposition, idiosyncracies all vary!
Course I cannot give advice on
every medicine you may use - but this is just to give you idea as to what to ask and tell GP.
But general and common:
ANTIHISTAMINES
Tell your doctor or pharmacist of other medications and if you suffer from high blood pressure, liver/kidney disorder, thyroid, urinary, pregnancy, glaucoma, prostate problems - can be decisive in generic brand to use.
Common side effects - Constipation, the runs, dizziness, drowsiness, nausea, vomiting (rarer but not unknown - fatigue, confusion, decreased sexual desire, irregular heartbeat, vision changes)
Although less frequent with generics such as astemizole and terfenadine
Alcohol can increased side-effects of dizziness and drowsiness and can even induce severe sedation.
Other interactions: Antidepressants, axiolytics, other antihistamines (Of same generic group), tranquillizers INCREASES the effects of the antihistamine. Likewise the antihistamine on these medications!
Antacids will DECREASE the effects (small drop of Andrews may help with the nausea and drowsiness to some extent!

)
Anti-inflammatories, oestrogen, progesteron and oral contraceptives will be compromised in effect by antihistamine.
Bear in mind that atihistamine will deplete Vit C (which is natural antihistamine in property and that excessive intake of alkaline foods (lettuce, tomataoes, potatoes, cream, fruits, etc - but most of these convert to more acid if you chew properly!) may decrease the excretion of these drugs and lead to overdose
ANTACID
generic - aluminium hydroxide
Doctor/pharmacist need to know if you have acute stomach pain, constipation, runs, hernia, liver/kidney, stomach bleed, pregnancy, prostate, retain urine. Inform of any other medication as well.
Overdose can cause fatigue, dental problems and if you do suffer for any drowsiness - do not drive!
Side effects are very rare but palpitations, dry mouth blurred vision with Kolanticon brands
if you are on betablockers - contact doctor/pharmacist as reactions are unpredictable!
Chew food properly as you need to create the enzyme ptyalin in your saliva. Antacids deplete Vits A, C and B complex and disturb phosphorus, magnesium anc calcium levels as well!
ANTIDEPRESSANTS
Large trolley load of these - you need to tell your doctor everything. They can cause drowsiness, dizziness, low blood pressure, fever, sweat, dental problems, vision disorders, and you certainly would not "feel the speed" effectively in a car. Has even caused some aggression in some cases.
React unpredictably with other antidepressant brands, analgesics, aspirins, anticoagulants (warfarin etc) and increase side effects of antihistamines and decongestants. (Aggression being one side effect)
ANTIVIRALS
(Zovirax, Lemsips et al)
Blurred vision can occur if medications are mixed with alcohol, and if the medication is generic of amantadine (see label!)
ASPIRINS
Again reacts badly with alcohols (sedative) and and anticoagulents (because if is blood thinner itself!) Can cause respiratory problem. You also need to tell your dentist if you have taken aspirin - especially if you are having tooth removed!
Greatest danger is taking with other medicines by mistake. If you mix aspirin brands - leads to ulcers and enteric coated aspirin should not be taken with antacid on this basis. Van cause deafneess and ringing in the ears - not good when driving as you do not hear the BiB's sirens when he is doing you for speeding! Also - the stomach upset - you would speed to get to nearest loo!
ANTI-INFLAMMATORIES
(Ibuprofen, Nurofen, Benoral, naproxen, ketoprofen etc.)
Need to mention heart disease, eye disorders, peptic ulcer, blood pressure, pregnancy, pyschological disorders, bowel problems, urine problem, digestion. If you are having treatment which means anaesthesia - let them know! Reacts!
Avoid indigestion remedies, sunbathing (you burn!). Can cause drowsiness if alcohol is consumed, but most common side effects are digestive! Relatively safe to drive though!
PARACETAMOL
(Solpadeine, Disprol, Disprin, Calpol, co-codamo)
Drowsiness and dizziness are very common side effects as is vomiting. Alcohol should be avoided completely with these medicines - risk of severe liver damage.
Taken with sleeping pill, barbiturates and alcohol - reactions unpredictable.,
Effects of contraceptives (side effects and effectiveness) are affected by paracetamols and aspirins and antinauseants increase the side effects and the effectiveness of this group.
Too much Vit C and drug is slow to excrete. Also amino acids will reduce liver damage caused by this medicine.
These are just some of the very common items in use by many drivers. There are so many - so ask the doc, the chemist and mention anything you think is relevant!
WildCat (posting with permission as probably relevant information!)