As a rule we recommend against crushing medications after weight loss surgery. There are some larger pills, however, that can be difficult to get down in the early days when there is still some swelling present. There are many medications, however, that should NOT be crushed under any circumstances. Generally speaking, these are extended release medications that have a special coating to cause the drug to be released slowly over a period of time. Probably the most common medication like this is Prilosec. If you crush it, the coating is destroyed and the medication will be absorbed all at once. In the case of Prilosec, this would likely cause little harm, however this is not the case with all medications and in some instances life-threatening side effects could occur.
We always recommend checking with your pharmacist, however you can also refer to this fairly comprehensive list of medications that should not be crushed under any circumstance!