Dalia Hernandez was admitted to Northeast Baptist Hospital in San Antonio, Texas at the end of 2011 for foot surgery. Hernandez, 73, suffered from kidney disease and was on dialysis. After the surgery, her kidney doctor decided to change a prescription for 10 millimoles of potassium to one for 20 millimoles.

But rather than write a new order, the doctor wrote the “2” over the “1” on the existing order. The nurse and pharmacist read the order as 120 millimoles of potassium, a fatal dose. Hernandez started receiving the drug intravenously at 3:38 a.m. By 7:50 a.m. she was having difficulty breathing. Less than half an hour later, she was dead.

“We see a lot of medication errors,” said Marynell Maloney, a San Antonio-based trial attorney who represented Hernandez’ daughters in a suit against the hospital and doctor. “This one was pretty dramatic because it was so lethal.”

At least 1.5 million patients are injured each year as a result of medication errors, many of which are preventable, according to a 2006 Institute of Medicine Study. A more recent advisory from the Food and Drug Administration found medication errors kill at least one person every day and injure approximately 1.3 million Americans annually.

Doctors’ messy handwriting plays a role in creating medical errors, and so do abbreviations, drugs with similar names and ambiguous decimal points.

Related: 6 Mistakes People Make When Taking Medication

Abbreviation woes

The Institute for Safe Medication Practices (ISMP), a nonprofit devoted to preventing medication errors, maintains a long list of error-prone abbreviations, symbols and dose designations.The institute says doctors should never use them because they are so often misinterpreted. Some examples:

  • Pharmacists often mistake QOD (meaning you should take your medicine “every other day”) for QD (daily) or QID (four times daily).
  • BT (meaning “take at ‘bedtime’”) is sometimes confused with BID (take “twice daily”).
  • The symbol U (meaning “unit”) may be read as the number “0” or “4,” which could cause you to get 10 times as much of the medication as you should.
  • The abbreviation “ss” for “sliding scale” is often mistaken for “55.”
  • "Per os" refers to medication to be taken by mouth (orally), but a pharmacist may read it as medication for the left eye ("OS," or oculus sinister).

As you can imagine, taking a drug four times a day instead of every other day or getting 10 times the amount your doctor intended may have very unwanted consequences

The devil is in the decimals — and the handwriting

Doctors’ handwritten scrawls have led pharmacists to confuse drugs with similar names, such as Flomax and Volmax, sometimes with deadly consequences, according to the ISMP.

When writing a prescription, the group suggests doctors never use a decimal point and zero after a number, which can inadvertently increase the dose ten times over if the decimal point isn’t obvious.However, if a doctor is writing a number such as “.9,” the ISMP suggests using a zero before the decimal to be as clear as possible.

Medication errors “remain a huge issue,” says Allen Vaida, PharmD, executive vice president of the ISMP. The state of Massachusetts has moved to require e-prescribing systems, which eliminate handwriting errors. Other states are expected to follow.

Related: Surprising Risks of Over-The-Counter Pain Meds

What you can do

The Institute of Medicine, the ISMP and other organizations suggest taking these steps to minimize dangerous mix-ups.

Share your drugs (with your doctor, that is). List all the prescription drugs, nonprescription drugs and supplements you take and share that list with your doctor during every visit. Or simply gather them all in a bag and bring in the bag.

Ask for a prescription written in plain English. Ask your doctor to please use his tablet or iPad to key in the name of the drug (brand and generic, if available), what it is for, its dosage and how often to take it — without abbreviations. If your doctor wants to use Latin or abbreviations for the pharmacist, ask if she can quickly key in everything out for you in plain English on another sheet.

Compare your prescription with the pharmacy’s. This is when your prescription in plain English will come in handy: Make sure the drug and the directions for use you receive from the pharmacy are the same as those written by your doctor. (In fact, an article in American Family Physicians encourages physicians to use plain English in their prescriptions exactly for this reason: so patients can compare their doctor’s instructions with those from the pharmacist.)

Get a printout of all your medications and their dosages ordered by doctors when you’re in the hospital. Tape this to the wall next to your bed and update it any time things change. With your list handy, ask nurses and doctors to explain what they’re giving you (and at what dosage) before you accept any pill, liquid or cream or have another substance added to your IV line.

Bring a medical advocate. If you are recovering from surgery and too groggy to monitor your medication, have a loved one or medical advocate present when you talk with the doctor about his plans for your medication.

This may all take some effort on your part, but it could save you or your loved one from a very costly mistake.

Related: Foods to Avoid When You're on Medication

Daniel S. Levine is an award-winning journalist who heads the Levine Media Group and hosts The Bio Report and RARECast podcasts. He was an editor of The Burrill Report and worked for the Oakland Tribune, Adweek, the San Francisco Business Times and other publications.