Thai traffic law does not have a list of medicines you may not drive on. It has two broader provisions instead. Section 43(1) prohibits driving while unfit to do so, section 43(2) prohibits driving while under the influence of alcohol or another intoxicant, and section 43 bis prohibits driving after consuming narcotics or psychotropic substances that the Commissioner-General of the Royal Thai Police has scheduled by publication in the Government Gazette. A prescription explains why something is in your system. It does not authorise driving while it is affecting you.
For most visitors the practical question is not a legal one at all. It is whether the tablet you took for a cold, a stomach upset or a sore back has quietly taken a fraction off your reaction time on a scooter, in traffic that gives you very little margin.
- Governing ruless.43(1) unfit to drive; s.43(2) intoxicant; s.43 bis scheduled substances
- Published list of medicinesSet by Gazette announcement, not in the Act
- Officer's powerMay require an examination; a negative preliminary test lets you continue
- PrescriptionExplains the substance; does not permit impaired driving
The legal position, stated carefully
Section 43 bis works by reference to a list that is made administratively. The substances it covers are those prescribed by the Commissioner-General by Gazette publication, which means the content of the prohibition can change without any change to the Act. We have not been able to obtain and verify the current list, and we are not going to publish a partial or guessed version of it — getting that wrong for a reader on prescription medication would be a serious thing to do.
What is clear is the structure. If a medicine is on that list, driving after taking it engages section 157/1, whose penalty is built as one third more than the penalty under the narcotics or psychotropic substances law for the substance concerned, plus suspension of the licence for at least six months or revocation. If a medicine is not on the list but has left you unfit to drive, sections 43(1) and 43(2) are still available: the "other intoxicant" wording in 43(2) is deliberately broad, and impairment is assessed by the officer and then by the court rather than by any published measurement.
What an officer can do
Under section 142, where there is reasonable ground to believe a driver is unfit under section 43(1) or intoxicated under section 43(2), the officer shall order a test. Under section 43 bis, competent officers may examine drivers of prescribed categories of vehicle for narcotics and psychotropic substances, and where a preliminary test is negative the driver may continue. Refusing that examination allows detention until it is completed and is itself an offence carrying a fine of up to 1,000 baht.
The medicines that actually cause trouble
Set the law aside for a moment, because the far more common problem is a mild, unnoticed impairment on a machine that punishes it. The categories that most often catch travellers are ordinary ones:
- Sedating antihistamines, in cold-and-flu combinations and in anti-itch preparations for bites and rashes. These are the classic culprit: sold everywhere, taken casually, and genuinely sedating for many people.
- Motion-sickness and anti-nausea medicines, which a lot of people take before a ferry to the islands and then ride a scooter on arrival.
- Codeine-containing or other opioid painkillers, including those taken after a minor crash.
- Sleeping tablets and sedatives, whose effect can persist well into the next morning.
- Anything new that you started this week, because the first few doses are when unexpected effects appear.
Two Thai-specific factors amplify all of it. Heat and dehydration make drowsiness worse and arrive faster than most visitors expect — sunburn and dehydration on a motorbike covers the mechanics. And the riding itself is more demanding than at home: a rented scooter you have used for two days, left-hand traffic, and the constant unsignalled negotiation described in right of way in Thailand. A ten per cent reduction in attention costs more here than on a familiar commute.
Read the packet, not the internet. If the leaflet says the medicine may cause drowsiness or advises against operating machinery, take that at face value on your first day of using it, and do not ride until you know how it affects you. That advice is dull and it is also the whole of what most readers need.
Thai pharmacies work differently
Visitors consistently report that Thai pharmacies dispense over the counter a number of things that would need a prescription in the UK, Ireland, Australia or much of Europe. Whatever the arrangements are, one consequence is worth stating plainly: the ease with which you obtained something says nothing about how it will affect your driving, and nothing about whether it is scheduled under section 43 bis.
If you are buying medicine in Thailand and intend to drive, the useful questions to ask at the counter are whether it causes drowsiness and whether there is a non-sedating alternative. Thai pharmacists in tourist areas commonly speak enough English for that exchange, and a non-sedating option usually exists.
Bringing your own medication into Thailand
This is a customs and health-regulation question rather than a traffic one, and the rules on personal medication — what may be brought in, in what quantity, and with what documentation — are administered by the Thai Food and Drug Administration. We are not going to summarise them here, because getting a quantity limit wrong for someone carrying a controlled prescription would matter a great deal and the position varies by substance.
The general travel practice is the same everywhere and is worth following: keep medicines in their original labelled packaging, carry a copy of the prescription or a letter from the prescribing doctor, and check with the Thai FDA or your own embassy before travelling if any of your medication is a controlled substance at home.
If you are stopped
A routine Thai checkpoint is a licence-and-helmet check and rarely goes further; police checkpoints in Thailand describes the ordinary version. Medication becomes relevant in two situations: where you appear impaired and the breath test is clear, and after a collision.
If you are asked about medication, the sensible approach is the straightforward one — say what you have taken and when, and show the packaging if you have it. Volunteering an accurate account is a better position than being found with unexplained medicine and no story. If language is a barrier, the tourist police on 1155 will translate, and your rights as a foreign driver in Thailand sets out where a traffic stop ends and something more serious begins.
Insurance follows the same logic as it does for alcohol and drugs. A policy that excludes driving while impaired does not care whether the impairment was prescribed. If a claim turns on your fitness to drive, the medication will come up. Does travel insurance cover driving in Thailand? is the page to read before you rely on a policy you have not opened.
Frequently asked questions
Is there a list of medicines I cannot drive on in Thailand?
There is a list of scheduled narcotics and psychotropic substances made by Gazette announcement under section 43 bis, but it is not reproduced in the Act and we could not verify its current contents. Rather than publish a guess, we would rather tell you it exists and that the general unfitness and intoxicant provisions apply regardless of what is on it.
Does a doctor's letter protect me at a roadside stop?
It explains the medicine, which is useful, but it does not authorise driving while impaired. The offence in section 43(2) is about your condition, not your paperwork.
Can I be drug-tested for prescription medication?
The examination power in section 43 bis is aimed at scheduled narcotics and psychotropic substances. Whether a specific prescription medicine registers on a preliminary test is a question about the device rather than about the law, and we do not have a source describing the devices Thai police use.
I took a cold tablet and then had one beer. Where does that leave me?
Legally, on the alcohol limit that applies to you — 50 mg%, or 20 mg% if you are in the group described in Thailand's lower drink-drive limit, which includes anyone riding a category their licence does not cover. Practically, sedating medicines and alcohol together produce more impairment than either alone. It is a bad combination before riding a scooter in unfamiliar traffic.
What about medication taken after a crash?
Painkillers given at a Thai hospital can be strong, and riding or driving afterwards is a genuine risk. Arrange a lift or a taxi and collect the vehicle later. What to do after a road accident in Thailand covers the rest of that day.
Does this apply to cyclists?
Section 84 of the Act applies a large part of the driving rules to cyclists, and the general prohibition on driving while intoxicated sits among the driving provisions. Treat it as covered — and in Thai traffic, treat impaired cycling as at least as risky as impaired riding. See how to cycle safely in Thai traffic.