Two different things are going on
Food mostly changes how quickly a dose gets going, so when it kicks in and when it reaches its highest. Vitamin C and antacids matter for a different reason. They change how acidic your urine is, and for amphetamine that changes how fast your body clears it, which is how long it lasts.
None of this works the same way for every medicine. What's true for Elvanse isn't true for Concerta XL, so this page goes medicine by medicine and quotes each one's Summary of Product Characteristics (SmPC), the official prescribing information for that medicine.
Food and when a dose gets going
Here's what each SmPC says about food. None of it is a reason to change how you take your medicine without talking to your prescriber or pharmacist first.
| Medicine | What its SmPC says about food |
|---|---|
| Elvanselisdexamfetamine | Can be taken with or without food. A high-fat meal doesn't change how much is absorbed, but it moves the highest level about an hour later, from 3.8 to 4.7 hours after the dose. |
| Concerta XLmethylphenidate | Can be taken with or without food. No difference was found after a high-fat breakfast compared with an empty stomach. |
| Medikinet XLmethylphenidate | Has to be taken with or after a meal and shouldn't be taken without food, which its SmPC says risks dose dumping, where too much is released at once. A high-fat meal delays absorption by about an hour and a half. |
| Equasym XLmethylphenidate | Given in the morning before breakfast. A high-fat meal delays absorption by about an hour and raises the highest level by about 30%. |
| Ritalin XLmethylphenidate | Can be taken with or without food. Food slows absorption, so the highest level comes about an hour later. |
| Ritalin and Medikinet tabletsmethylphenidate | Ritalin can be taken with or without food, and food raised its highest level by 23%. Medikinet tablets are taken with or after meals, and food speeds up absorption without changing the total absorbed. |
| DexamfetamineAmfexa | The effect of food hasn't been studied, so its SmPC asks for doses at the same times relative to meals each day, preferably with or just after them. |
Vitamin C and amphetamine
Applies to Elvanse and dexamfetamine
Almost all of an Elvanse dose leaves your body in urine, and how much amfetamine leaves depends on how acidic that urine is. Elvanse's SmPC says ascorbic acid, which is vitamin C, and other things that acidify urine increase how much amfetamine is excreted and shorten its half-life.
The dexamfetamine SmPC goes further. Ascorbic acid and fruit juices lower how much is absorbed from the gut, things that acidify urine increase how much is excreted, and both lower blood levels and how well it works. It also gives the size of the swing. In acidic urine about 80% can leave unchanged within 24 hours, and in alkaline urine only 2 to 3% leaves as free amfetamine.
If you take vitamin C, or drink a lot of fruit juice, your pharmacist can tell you whether it's likely to matter for your dose.
Antacids and alkalinisers
Applies to Elvanse and dexamfetamine
The opposite happens when urine is more alkaline. Elvanse's SmPC says sodium bicarbonate and other things that alkalinise urine reduce how much amfetamine is excreted and extend its half-life.
The dexamfetamine SmPC names sodium bicarbonate as increasing absorption from the gut, and acetazolamide and some thiazide diuretics as reducing excretion. Both raise blood levels and strengthen its effects.
Neither SmPC mentions proton pump inhibitors such as omeprazole. If you take anything for heartburn or indigestion regularly, ask your pharmacist how it sits with your medicine.
Methylphenidate works differently
Applies to Concerta XL, Medikinet, Ritalin and Equasym XL
Methylphenidate isn't cleared the way amphetamine is. The Ritalin SmPC says most of a dose leaves in urine as ritalinic acid, which it describes as probably pH-independent. Concerta XL's SmPC adds that methylphenidate isn't metabolised by cytochrome P450 to a clinically relevant extent, so medicines that speed up or slow down those liver enzymes aren't expected to change it. The urine acidity effect described for amphetamine doesn't appear in these SmPCs.
Medikinet XL is the exception for antacids, and it's about the stomach. Its SmPC says it must not be taken together with H2 receptor blockers, proton pump inhibitors or antacids, because they could make it release the whole amount faster, and it lists treatment with any of them among the reasons it shouldn't be used.
Alcohol matters too. Concerta XL's SmPC says alcohol concentrations above 10% increase the cumulative release of methylphenidate, and the methylphenidate SmPCs advise against drinking during treatment. There's more in our guide to ADHD medication and alcohol.
Talk to your prescriber or pharmacist
Section 4.5 of every SmPC lists what the medicine interacts with, and your pharmacist can check it against everything else you take, including supplements and anything you buy over the counter. Please don't change what you eat, when you take your dose or what you take alongside it because of this page.
If your dose seems to run shorter or longer on certain days, a record of those days gives your prescriber something to work with at your next review.
How ADHDose records it
The dose sheet asks two things. The first is "Did you eat before taking it?", with Empty stomach, Light snack or With a meal, and you can skip it if you're not sure. Your answer moves your curve by your medicine's own published figure. With Elvanse, With a meal puts your highest level about an hour later. With Concerta XL nothing moves, because its SmPC found no difference.
The second is "Did you take any supplements today?", with Vitamin C and Antacid / PPI. If you take an amphetamine, the sheet adds "Vitamin C can shorten how long it lasts, antacids can stretch it out."
Those answers are recorded, and they don't change the curve. With Pro, Insights compares how long your dose lasted on days you logged them against days you didn't. It needs at least seven days with a wear-off time, three on each side, and it only shows a difference of half an hour or more. More on how the curve learns your own times is in kicks in, peaks, wears off.
Log what you ate and what you took
ADHDose moves your curve for food by your medicine's own published figure, and keeps a note of vitamin C and antacids so you can see whether your days run differently.
Download free →Written by Matt, founder of ADHDose. Last updated October 2026.
Every medical statement on this page comes from a medicine's Summary of Product Characteristics, listed below. What the dose sheet asks is quoted from the app itself. The page has not been through clinical review, so use it to prepare for a conversation with your prescriber or pharmacist, never in place of one.
- Elvanse Summary of Product Characteristics, section 4.2 for taking it with or without food, section 4.5 for urine acidity and alkalinity changing amfetamine's excretion and half-life, and section 5.2 for the food effect on its highest level and urinary excretion. medicines.org.uk/emc/product/14091/smpc
- Amfexa (dexamfetamine) Summary of Product Characteristics, section 4.2 for taking it at the same times relative to meals, section 4.5 for acidifying and alkalinising agents, and section 5.2 for pH-dependent elimination. medicines.org.uk/emc/product/7403/smpc
- Concerta XL Summary of Product Characteristics, section 4.2 for food, section 4.5 for cytochrome P450 and alcohol, and section 5.2 for the high-fat breakfast study. medicines.org.uk/emc/product/6872/smpc
- Medikinet XL Summary of Product Characteristics, section 4.2 for taking it with or after a meal, sections 4.3 and 4.5 for H2 receptor blockers, proton pump inhibitors and antacids, and section 5.2 for the food delay and dose dumping. medicines.org.uk/emc/product/8238/smpc
- Medikinet tablets Summary of Product Characteristics, sections 4.2 and 5.2, for taking them with or after meals and the food effect on absorption. medicines.org.uk/emc/product/8235/smpc
- Equasym XL Summary of Product Characteristics, sections 4.2 and 5.2, for taking it before breakfast and the high-fat meal effect. medicines.org.uk/emc/product/3887/smpc
- Ritalin XL Summary of Product Characteristics, sections 4.2 and 5.2, for taking it with or without food and the hour's delay with food. medicines.org.uk/emc/product/15966/smpc
- Ritalin Summary of Product Characteristics, section 4.2 for food, and section 5.2 for the food effect and excretion as ritalinic acid. medicines.org.uk/emc/product/15987/smpc