While generics are suitable for most conditions, 2026 medical guidelines advise caution with Narrow Therapeutic Index (NTI) drugs. These are medications where a tiny difference in dose can be the difference between a "cure" and "toxicity."

Examples include:

• Warfarin (Blood thinner)

• Levothyroxine (Thyroid hormone)

• Lithium (Mood stabilizer)

• Seizure medications

For these specific drugs, doctors in 2026 often recommend staying on the same manufacturer’s version (whether brand or a specific generic) to avoid even the minor 3–5% variations in absorption that can occur when switching brands.